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.
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.