For a long time, it seemed like modifications were on the way out. Very few games supported mods during the last generation, save for a handful of shooters, and a number of strategy and RPG titles. That is changing though, thanks to a rebound in the focus on user generated content. Even if a game is a completely solo experience, you can play levels or experience new content made by other gamers.
User generated content is the lifeblood of many older games. Tron 2.0 and Skyrim both got fan expansion packs in the past three years, well after their publishers had moved on. Mods are free DLC that developers don't have to spend a dime on. Whether or not you think mods should be commercially released is another debate, but you can't deny the popularity of modding. Some developers even use mods as ways of finding the best new talent to hire for their next project.
As development tools become more user-friendly, and in-game toolsets get more powerful, it stands to reason that user generated content needs to be taken more seriously as a means of online content.