Wouldn't shortening the front also shorten it for the enemy?

You have to remember automatic weapons were heavy and hard to move, so the defender had that advantage, and they also had time to leave traps in the places they left behind.

There were times when ground was given up because they enemy was on a hill in front of them.

Not Proportionally

Not only does shortening the front allow you to choose more defensible positions, but if you do it correctly it can shorten the front for you more than your opponent.

For example, if your frontline is a curved shape, the "outer" part of the curve is longer than the inner curve. So lets say theoretically the current front is a straight line 10,000 yards long, and it takes 10 battalions to cover it. (1 battalion per 1,000 yards was about average for mid-war "normal" sectors) If you pull back to, say, a curve of 7,000 yards, you've freed up 3 of your battalions. But the enemy is having to cover the outer edge of the curve, which might be 8,000 yards, thus only freeing up 2 of their battalions. Plus you have the added benefits of having chosen the best positions you can, (good lines of sight, higher ground, interior lines, etc) which means you might now be able to hold 1,200 yards with a battalion. So that frees up another battalion-ish if men. Meanwhile your enemy is in a proportionally "worse" line, and may feel they need more men to cover a given frontage in the new area. Say 800 yards per battalion. So now they need 10 battalions to cover 8,000 yards, the exact same amount they needed to cover the original 10,000 yard line while you've freed up 4 battalions!

The above is just a theoretical example of course, and a fairly extreme one as I have the traditional history major's problems with complex math. But the principle remains the same. Even if you withdraw backwards to another 10km long line, if that line is more outwardly curved than the previous one, the enemy will need to add troops to maintain his normal frontage. You also don't want to make things TOO curved, or your get a salient that can be "snipped off" surrounded, and forced to surrender.

If (as the germans did) you decide to go over to the strategic defensive, shortening the lines also means you have to cover less frontage with any given reserve. Remember an attacker starts out concentrated, whereas a defender must (theoretically) defend all points equally well. A shortened front means your reserves can react more quickly when there's a sign of a potential breakthrough. It also allows your front to be proportionally stronger. But for the attacker a shortened front means fewer places to look for potential weakness.

tl;dr: Your own interest might be to shorten the front line, no matter if it is good for the ennemy as well.

Answer:

As other answers have pointed out, there might be configurations in which shortening the front offers you more freed troops than to your ennemy, and might give you a better tactical situation.

But sometimes, and this happened a lot during WW1, you are looking with this manoeuver only to save your exhausted army from breakdown. You want to freed troops so that they can rest, and then replace the ones who stayed on the shorten front. Thus, after some weeks, all men have gone through a round of rest. This is a crucial gain if you are in a stalemate on the front for tactical resons (see WW1), but your exhausted army might just suffer from breakdown of morale (see again WW1, uprisings of 1917 in French army).

The second advantage is that, if you defend, the shortened front gives you reserve. Reserve are useful because they can move on a safe ground to the important point, while the reserves of the attacking ennemy, while also obtained by shortening the front, will have to walk on a fighting ground. Since attacking needs more men than defending (usually), and since this is true with reserves, then shortening the front gives you reserves of X value while it gives the ennemy reserves of X/2 value (if attacker needs 2 times more men than the defender).

Another aspect is the concept of local superiority and the old adage that it takes 3x troops for an attacker to break through.

A very long front allows a sufficiently deceptive attacker the opportunity to concentrate their forces and attain a sufficient local advantage, away from immediate defensive reinforcements.

A shorter front allows the defender to more quickly reinforce any position under threat, thus negating local superiority and bringing that area back in balance favoring the defense.

Imagine you're in your house in a horror movie and you know there's a murderer outside trying to get in. You do have a weapon, but you live in a rural area and can't get help quickly. The more windows and doors you can seal off, the better. But if you have to confront the murderer, you close off everything but the front door, and then you wait by it with your weapon ready. Perhaps you even leave it unlocked and wait behind it!

This is a form of "shortening the line". It's true that by blocking the other windows and doors you reduce the attack surface for the murderer just as much as for yourself, yet somehow it's still intuitively desirable.

There are a few incentives for defenders to shorten the line:

• Defenders can fortify. Fortifying an area is typically more costly than mobilization against an unfortified area. Hence, reducing the amount of fortification necessary benefits the defender more than the attacker. (In World War One this still mattered, whereas in later wars fortifications could be matched by long-range artillery installations or simply bypassed from above. However, secondary meanings of "fortification" might still apply today.)

• Defenders gain intelligence. A limitation on the possible enemy movements translates to an increase in knowledge of where they'll come from. (This is you behind the unlocked front door.) You might argue that this also means the attacker knows where you'll be waiting for them, but consider that you can distribute your forces however you like behind the line, whereas all the attacker's forces must pass through the line.

• Defenders know or have time to study the lay of the land better, and if they're taking the initiative to shorten the line, they can shorten it to the most defensible section. This seems to have been a part of the logic behind Operation Alberich specifically:

Fourteen fewer German divisions were needed for line holding... The new defences were built on a reverse slope with positions behind the defences, from which artillery observers could see the front position, experience having showed that infantry equipped with machine-guns needed a field of fire only a few hundred yards/meters deep. [Wikipedia]

• Bottlenecking slows the pace of war, and that can help by allowing you to prepare behind it or wear out the enemy's resources. Of course, that can go to either side's advantage depending on who's in a better position to hold out and reinforce. Presumably a defender who shortens the line does so in the belief that they can replenish and fortify behind the line while the attacker is extended for longer than they hoped.