If you play a lot of games on PC, you've probably installed at least a handful of mods before. And if you haven't, you're missing out.
Mods can do a lot for the games you know and love - revitalize the graphics, add hours of new gameplay, or even make wholly new games on the same engine. They can range from lovingly crafted tweaks meant to feel right at home in the base game to silly item or creature replacements and god modes. What's more, they're easier than ever to install, whether it's using mod managers like nexus or simply doing it manually by following a video guide.
The truly fantastic thing about mods is that they allow you to customize your experience with a game, completely and utterly. Obviously you may be limited to the mods that exist if you can't make them yourself, but what you choose to install out of any given number of options is still all up to you. If you were to play Skyrim, for example, and decide you'd like to make your character a master chef, there are mods to install to make cooking more robust. If you wanted to play Dragon Age: Origins, but really don't like a companion's outfit, you could install a mod to change it. It's that easy!
Since mods are creations made by fans, they also have the capacity to exist outside of what's feasible for some developers and to improve upon games long considered 'past their prime'. As a good example, the Final Fantasy 6 port on Steam is, put gently, not very good looking. That's because the Steam version was actually meant for mobile devices and then moved to PC afterwards, and the sprites were not only changed from the original (although they may be just fine on their own) but showed up poorly due to the way they were filtered.
If you showed Steam's FFVI to someone who'd liked the original, they'd probably be frustrated with the way it looked and played. Because the mobile UI frustrated people, mods were made to restore it to something more like the original, or at least nicer looking. Many mods like this exist for old games, in fact - just this week, modders finished a beautiful fanmade remaster of Final Fantasy IX.
And that's only the surface level of things mods can improve. Sometimes, games can be changed radically (and for the better) with just a handful of truly fantastic mods. Ask anyone who's modded Skyrim before and they'll likely have lists upon lists of can't-live-without changes: some people swear by Apocalypse, a mod that adds over a hundred interesting spells to the game. Fans of Rimworld often adore Prepare Carefully for creating custom colonies, and Hospitality for making the in-world faction relations more engaging.
Implementing new mechanics into a game you've already spent a lot of time with is a good way to add limitless replay value. Turning Minecraft into a Pokemon game or Skyrim into a survival sim opens up new possibilities in the way those games are played. And, finally - if there's something you think could be changed in a game to make it more enjoyable, why not go change it? It doesn't generally cost anything but a bit of computer space to install a mod, and the vanilla game will always be there if you change your mind.
Blizzard QA staff are showing solidarity with their Raven colleagues by walking off the job today.