For a non-believer, what good does realizing impermanence do?

For these kinds of reason I think the question is still unclear (even though you replaced "atheist" with "non-believer in the rest of Buddhism"):

• According to your definition of "realise", can you "realise impermanence" but still be unwise, fail to benefit, and stay attached, heedless, suffering?

• Would your definition of "thorough" be that any sufficiently "thorough" necessarily understanding implies every other "beneficial" realisation?

• What does "a non-believer in the rest of Buddhism" mean ... does it mean that you don't "believe in" the four noble truths, for example?

• Conversely you said "at all", which might imply that even the faintest fleeting possibility of benefit should count as a "yes" answer.

• Does the third noble truth imply that everyone benefits from impermanence, whether or not they even realise it?

I guess, depending on what you mean by "realising", it's not sufficient, and perhaps the answer is no: "impermanence" is not mentioned in Itivuttaka: The Group of Ones, which mentions more than a dozen other "one thing"s as being sufficient

Understanding impermanence is important for non-Buddhists.

You need to understand and realize that your life, youth, health, good physique, beauty, good looks, wealth, loved ones, friends, good fortune, happiness, reputation, career, skills, intelligence, ideologies etc. will never last forever. You can lose them suddenly at anytime or you can lose them gradually.

You need to understand and realize that you will grow older, you will get health problems, and eventually you will die.

You need to understand and realize that sensual pleasures that you enjoy and relish e.g. your favourite foods, tourism, music etc. may at some point be unavailable to you for any reason, and this might frustrate you.

You need to understand and realize that human companionship that you enjoy with others may suddenly vanish and you may become alone and lonely.

This will help you accept situations more easily without denial or depression, for e.g. if your spouse suddenly passed away or if you get diagnosed with a terminal illness or if your all possessions got destroyed in a natural disaster. Think of it as fostering good psychology.

Understanding impermanence will give you an underlying feeling of dissatisfaction. You will come to see that if you depend on anything that's not permanent for your happiness, then it will ultimately not satisfy you.

This underlying feeling of dissatisfaction is the realization of the first noble truth that there is suffering. And with that, you would have your first step, to cross over from being a non-Buddhist, to becoming a Buddhist.

Realising impermanence is a big part towards ending dukkha. If you can see that all things must change then it'll be easier to see that these things don't bring happiness and you'll transition towards non-attachment. This will benefit you in this life.

Note that realising impermanence doens't necessarily mean you become unattached. For example, some people see and accept that they must die but this frightens them. You need to know impermanence and also see that impermanent things aren't worth clinging to.

Three words to help a secular accept of impermanence: Negative visualization, early retirement, and Hedonic adaptation.

Awareness of impermancence is also one of the basic pillars of Stoicism. Once you become aware of, and appreciate, impermanence you'll: find joy every day you get to spend with your children, you'll learn to appreciate your daily cup of coffee.

When you lose something, or break something, you'll have spent significant time understanding that the things was already broken or lost. Because you have internalized that the thing was going to pass away some day, you will not only have appreciated it more while you still had it, you have also spent time preparing yourself for its demise, significantly lessenning the suffering associated with the departure.

This day will end; what good have you done? This job will end; what good did it do you? Your life will end: what good did you do? Your children will die one day; what good did you do them?

Amidst all this morbid speculation actually comes an amazing liberation, coupled with a new take on life: make the most of what you have, love what you have, appreciate what you once had, and live now. Live now, 'cause you're already dying.

Here's a capitalist kicker: Once you start to realize the impermanence of stuff, you also get a much more realistic picture of what it's all actually worth. Not what it costs, but what its value is. If you google financial independence you'll find two basic approaches.

1. Start a company and hopefully make a boat load of money
2. Learn to live well on very little money and invest your surplus.

Approach number two relies heavily on realizing the difference between price and value. Meditating on the impermanence of everything is an amazing way to gauge that difference.

You see, you become hedonically adapted to everything, and so in time your life starts to suffer from inflation. You want newer cars, bigger homes, more stuff, more everything. Internalizing the impermanence of stuff is a great way to internalize the futility of owning all that..... crap. Hedonic adaptation robs us of our appreciation of sunrises, indoor plumbing, vaccines, a warm seat, a roof over our heads, company, electricity etc.

When you start meditating on the impermanence of everything you'll start to reverse that hedonic adaptation and start to actually LOVE a simple life. This is part of the reason why those 'weird' monks and minimalists are so eerily happy with their lack of stuff, while 'the rest of us' are busy stuffing our homes.

Accept impermanence and you'll learn to love what you have, learn to release your fears when you're stuck in a bad spot (this, too, shall pass), you'll spend less money on things that you don't need in your life, you will get a better perspective on your day-to-day life, on and on.... Once you really grok it, it becomes a pillar of a good and robust life.