I think the most common use case of a non-compete is to prevent employees who have developed strong customer relationships during their time at a firm from leaving that firm and bringing those customers with them.
I think that these are specific examples of the same general case: there are some assets that are easy to "walk out" of a company with, and a non-compete is a way to deal with that fact.
Sometimes employers even know that they aren't enforceable but are still doing them "because our lawyers say so." Which is a standard negotiating tactic, but there you go.
I'm certainly not an expert on this, but I think what hnal943 said is the general motivation behind these types of agreements. The one "non-compete" agreement that I've signed in my career covered both non-competition and non-solicitation all in one, and I was using the term to cover all of that.
