Well, I think the majority of gamers should have their own reasons for playing mods or modified versions of a particular game application or software that is free-to-play. In my experience, most gamers (especially Android users) are always looking for a game that is more challenging and has better graphics or features than the original.
So, in this case, the mods of such games tend to offer much more than the basic game itself. Aside from that, mods can also be used to improve the gaming experience by fixing errors or glitches that are found in the original game software.