But on the top of my head, there can be a number reason why someone would give game-mods a go:

If a game is a few years old and have picked up a modding-scene, it is most likely someone have made an overhaul-mods, that is basically is a collection of popular mods. If you've either picked up an older game or just wanted to revisit one, one of those mods seem to be the way to go.

Except people who make these Overhauls seem to assume, that people seek more challenge out of the game.

I've been trying out some mods with the space-trade-sim Freelancer lately. More specifically the overhaul mods Crossfire and Discovery.

Both of these mods offer an expanded universe, with more ships, mechanics, improved graphics,the possibility to play with the mod online on their server... and enemy npc's that take FOREVER to take down because of "improved" AI - However, this just means that the have increased ability to heal themselves.

I've played about an hour of the mod Crossfire today before I got annoyed with this, and wasn't able to take out a single enemy in this frame of time. Be aware that the pay-off from any of these missions wouldn't have been correspondent to the weapon-costs used during battle. It's apparent that you start of with enough money on the game-server to buy some better weapons - however nothing in the system you start out in is expensive (or powerful) enough to match the enemies you meet in the same system. 

The enemies are in fact not more challenging - they are just more time-consuming to take down.

Personally I don't se the logic in adding a whole lot of ships and equipment to the gameplay, while effectively rendering a whole lot of ships and equipment useless from the start of the game. If you're trying to develop an online community around a mod, I wouldn't recommend alienating people who seek to play to... you know... just to have fun.

It seems to be a reoccurring theme among plenty of mods for several games. The mod Discovery for the same game is similar in that way, however I was able to take down some enemies. I just had to devote roughly 20 minutes per enemy - Since the amount of enemies that fly together in groups are largely the same as the original, you have to devote a lot of time to a single battle. I recall having played a mod for Pirates of Caribbean, which also added a whole lot of new and exciting enhancements... along with enemy ships that took forever to take down.

I'm currently waiting for Oblivion and looking to play this game with mods. I already have the game for PS3 but got very annoyed with how the level-progression is done. This is however something I've read that there are made mods to fix in different ways - Coupled with a mod that makes it possible to leave your alchemy-equipment at home for some home-brewing, this sounds like a sweet deal to me.

Now if I can just experience these enhancements without having to run into wolves 10 levels above my own level...