Lots of reasons.
-Long runs might be boring for some people
-Intervals help you improve more quickly than does distance running
-People can point to workouts and be more impressed with themselves rather than only doing distance runs
-random logical fallacies that are easy for the uninitiated to not understand, e.g. "If you run faster, you'll get faster" / "if the workout is tougher, it's better" / "workouts bring you to exhaustion faster and are therefore more efficient"
They would almost certainly be better off doing distance runs if they want to be better runners in the long term. But in the short term, or if they aren't overly concerned about their times, speed work is fine as long as you don't get injured from rushing into it.