I really hope you're ready for some philosophy.
Humans need entertainment and the ability to entertain themselves and others for the sheer purpose of life itself. This ties into competition, amongst other things, the inner machinations of the human mind. Without any purpose or substance within a human's life, the human loses interest in actually living to optimal potential. Because of this, the human drives to FIND the purpose and such that go along with the bonds that keep him living; thus, he finds whatever he can to make himself in the best position possible by competing with other humans, or just sticking to his own presence and enjoying himself. Without this, insanity ensues. The humans created entertainment out of an attempt to add variety to daily life instead of redundancy. Entertainment can be composed of: Watching something, doing something (both as actions), competition, and excitement.
That is why games exist; they create a virtual environment in which to provide for the key points of entertainment. SRB2 is part of the type of entertainment, "Video Games". WADs and SOCs are used to add even MORE variety in SRB2 to keep this game from becoming redundant, and allowing the people playing it to have a creative outlet, and be somewhat competitive.
There, you got your answer. Let's see how well you dodge this.