The cool thing about a lot of the mods is that they don't just add more content to the game, but they add whole new ways that the game can be played. In vanilla minecraft you can build all sorts of structures, and that's very cool and has a lot of room for a creative mind to explore. But then there are mods that add the ability to build automated factories. Or robots that you can program to do almost anything the player can, like gather resources. Or movable structures which allow you to build bases that can fly around the world. Or bees that you can selectively breed or even genetically engineer, which will produce useful resources for you.