Some of them also offer new Pokemons (called Fakemons). You can find all the Pokemons in the wild, new palettes, new regions, new music, and a complete overhaul of the story. It is just like playing a whole different Pokemon game. There are countless new things you can get with them. The Pokemon ROM hacks are a solution to all of this.
There are limited Pokemon availability, complex evolutionary mechanisms, and a bland storyline. However, the original game versions come with a lot of restrictions, limited features, and very little freedom.
