Just for experience. Almost anyone can make a hardware renderer with OpenGL nowadays, it's been made so easy. Programming a Quake equivalent with OpenGL today would mostly involve designing data structures and writing tools. My project allows me to get down to a really low level and do it all myself, and I really like that.

In fact since I've started this project my knowledge of computer programming has expanded dramatically in all directions. I've learned so much about 3D math, assembler, advanced C techniques (function pointers ftw), caching, the floating point unit, the MMX unit, and more in just a year. I love this kind of programming.