Empoleon, like a lot of pokemon, got Defog through the gen 4 HM.
These move tutors give it HA+Defog availability (in Empoleons case Defiant), and makes it legal for VGC with Defog and some other stuff, it's very neat.
Protean Greninja outclasses Ash-Greninja either way, so I don't care. I guess...