Mods made PC gaming what it is today.
For example, Valve released 'Halflife' in 1998 and while a solid game in its own right, it wasn't until the Counterstrike Mod was released did it really take off and significantly elevate sales of the base game. It ensured 'Halflife 2' go into production to be the first game released on Steam.
Would steam exist without the success of the Counterstrike mod?
I can't answer that question but the fact it can be asked at all demonstrates the influence of a single mod.