Those are typically coins that either try to fix something about Bitcoin that isn't broken or add something which no one wants. I've yet to see an alt coin that adds value on top of Bitcoin.
I'm all for their creation and existence though. Competition is useful.
Probably a good way to find bugs, as well.
And by developing them, people are gaining competence that is also useful within bitcoin. Just creating a trivial new blockchain is a very interesting exercise.