The "lock fare" option is useful if you want to keep your bank account balance/credit unaffected. A refund can take a while to process so you might be without that money for a week or so otherwise.
Since it isn’t completely useless the airline has no reason to remove it. It is easier for them to always offer it rather than to have rules about when to offer the "lock fare" option.
Also, if it is there people will use it and it is easy revenue for them.
The fare lock means you are certain you can buy the ticket (before the 24 hours is over) for the price you are seeing it now.
Prices of airplane tickets go up (and down).
If the price goes down, the "refundable if you cancel before the 1st flight in your trip" is interesting. You refund it and buy the ticket again. (I’m not sure if that’s allowed though)
If the price goes up in that 24 hours, you can still buy it for the price when you saw it. It has nothing to do with the refund.
Credit:stackoverflow.com‘