Why is a combination lock better than a key lock for a locker?

The reason many people prefer combination locks over key locks is that they don't require the user to carry a mechanical key. It's not inconceivable that you'll be moving in many different ways during a workout (in general) and that something as small as a key will fall out of your pocket or that any particular pocket won't get in the way of one or more activities.

From a security standpoint, locker locks are usually designed to keep most people away from other people's belongings in a reasonably supervised environment. It is certainly possible to bypass, manipulate, cut off or otherwise open a lock that does not belong to you, but it is likely that someone else will enter the locker room to give notice. You just need something to make getting into your locker troublesome enough so that others will notice that person grabbing your purse.