I believe the easiest way to understand why this is important is by considering functions that don't follow this rule.
You'd often straggle to give them a clear name. Function defines a concept in a language that you are creating. If your function is too large and it's doing too many things - you can't give it a name that would sharply define the concept. Making your language less expressive.
At a first glance this seems like it contradicts functions should be small. But that's not really the case - when you start following the underlying principle of separate intention from implementation - you'll notice your functions getting smaller. Usually ending up just a few lines long. So while the size in itself is not the point - keeping functions small can serve as a proxy/heuristic for the underlying principle.