pretty much what omega said.
More programming/graphics space so you can have more graphics and coding space.
The actual mmc5 mapper functions are only if a person knows what they are, and how they work and if they want to use them in their hack or not. (again only if that person actually has advanced knowledge already about the mapper)
And no it does not change megaman 1 (or any game) in any shape/way/form than it originally was outside of that. The game will still work 100% perfectly as it was before ever doing such thing.