For example: if you learned to ride a bike as a kid, you'll remember a moment where your balance just "locked in" and there you were riding for an arbitrary amount of time.
Now take twelve-bar blues on the piano: say you spend some weeks or months learning some bass patterns in your left hand. Maybe you learn a simple melody for the right hand. Then you put them together. Then you learn a few "riffs" for the right hand.
If you keep practicing consistently, at some point you develop your coordination in your left hand to the point that you've got a steady rhythm in the bass line. And you won't stop playing that bass line even if you mess up in the right hand.
No longer are you merely practicing the parts that go into playing the blues, you are practicing playing the blues by actually playing the blues. And each time you play a chorus, you have aural feedback you can use to decide whether the things you played in your right hand are worthwhile or undesirable. You can even play a thing which you did not intend that turns out to work, and use it later.
Essentially, you can now cycle through the blues form an arbitrary number of times. That's an epiphany, and it feels the same as the moment you learn to cycle an arbitrary number of times on a bicycle.
There are similar epiphanies when learning to construct a melody over a pentatonic scale, and an even stronger one when learning how to construct a melody over an octatonic scale. Those moments are so intense that a lot of musicians end up just stopping there because it's so much fun.