Yeah your right. But those graphics, upgraded textures, weapons looking more realistic, enviroment looking more awesome, more dynamic lightning, etc, they all are great bonuses.
ID were nice enough to give the people the DOOM map editor where you could creat your own maps to DOOM 98 with it's Wads. With that, everyone could mix and tinker around with the DOOM engine, and the result is many engines made for the DOOM wads, like Zdoom, GZdoom, DOOMSDAY, and so many more I forgot the name of. Zdoom and DOOMSDAY are the most popular though. Zdoom because it had those funn add-ons like simpsons theme, mario theme, and lots of other themes. Instead of fighting those imps you could be fighting a simpson character.
Edit: And of course Risen3D, how could I forget?