It's a pretty easy way to let ourselves (or someone else) off the hook. "Hey, you did your best."
But it fails to explain the improvement in the 100-meter dash. Or the way we're able to somehow summon more energy and more insight when there's a lot on the line. Or the tremendous amount of care and love we can bring to a fellow human who needs it.
By defining "our best" as the thing we did when we merely put a lot of effort into a task, I fear we're letting ourselves off the hook.
In fact, it might not require a lot of effort, but a ridiculous amount of effort, an unreasonable amount of preparation, a silly amount of focus... and even then, there might be a little bit left to give.
It's entirely possible that it's not worth the commitment or the risk or the fear to go that far along in creating something that's actually our best. But when we make that compromise, we should own it. "It's not worth doing my best" is actually more honest and powerful than failing while being sort of focused.