What's the Point of Modding FF6 on Different Platforms?

Many potential reasons, absolutely. Alot of things are limited by the SNES hardware (yep, even on an emulator). GBA has a few less hardcoded restrictions, little more room to add. Android version? No, just NO.

The PC version potentially (actually I'd risk saying absolute) doesn't have those restrictions.

What sort of stuff? Well easier to have more than 255 items, showing more than 19 sprites on the screen at once, music, umm... anything to do with VRAM, number of palettes for characters, hell maybe even PC sprite resolution.

So yeah plenty of reasons if you really want to go all out. At the same time, SNES still has the challenge factor, just how far can it go? As for basic stuff however, nah stick with the original.