"Shoot for the moon. Even if you miss, you'll land among the stars." - Unknown
It makes little sense to reach for the ridiculous with the predetermined idea that you will fail. Setting goals that are near to impossible to achieve can make you feel like a failure and demotivate yourself from self-growth and success. For example, as a writer, it would be complete nonsense to plan to be a #1 New York Times Bestselling author by this time next year--I could have a manuscript for a book done, or even have it ready for publication, but that seems a lot less likely if I intended on achieving the former, loftier goal in and of itself. I would never write again if I intended for my words to be "best-selling" because none of them would seem good enough. And goals should lead you to a place where you feel good enough.
You may want to become more fit, but you don't have to run a marathon. You may want to be more cultured, but you don't have to travel all across the world. Goals are based on marginal improvements that you make over time. In the exercise example, you would start with maybe 30 minutes a week, then 30 minutes a few times a week, and continue to make marginal improvements until you reach your goal of what it means to be fit.
Each successful step you take is a win on its own; in contrast, there isn't much of a straightforward path to "shooting the moon" - in the card game of hearts from which this phrase was taken, it is largely based on luck and typically you lose big time. "Shooting the moon" provides an excuse for failure that is external to yourself, it lets you push the responsibility on external forces. Reasonable improvements, on the other hand, are your own responsibility. If you don't make the time to exercise as planned, that's on you. If you don't become a marathon runner, you could blame that on any number of reasons.
A much more effective method to goal-achievement is to use the S.M.A.R.T. method, which stands for specific, measurable, achievable, relevant, and time-bound. This is a very popular and well-known tool available in many self-help resources. Shooting for some impossible, nebulous goal is clearly not an effective approach, yet I continue to see this quote plastered on walls, on social media, in books, etc. Why? Because you're provided with a method that allows you to feel good about failure and good about not striving to be the best you can be - after all, why bother with an impossible goal?
Of course, failure can be a valuable opportunity to learn and grow as a person; however, "shooting for the moon" is a near guarantee for failure without giving yourself a real shot. That's not fair to do to yourself. Besides, the assumption that "you'll land among the stars" is far-fetched - it's better to work smarter in order to get to that place.