The best way I can explain this to a layman will only work if you watch american football.

What is a “catch”?

We can call a catch a catch, but unless we go further and state things like “must have control of the ball”, “must have one foot inbounds” etc etc etc, we have an incomplete rule that will allow for things we did not intend. So we narrow the existing rule, because ostensibly it is what we meant, but we need to be more specific to communicate the full idea than we originally believed.

In basketball… What is a travel? Seems straightforward…