Why do I need a Launchpad?
You need a controller if you want to play instruments in your DAW, and really feel what you're playing. A mouse if ok for making music but if you want to do multiple things at once you need a controller.
A grid's laid out like Ableton Live's clips and scenes.
You can perform your Ableton session using loops and clips in real time, and easily see where everything is. You can play scenes - rows of clips or loops all at once, maybe for whole sections of a track, e.g. intros, drops etc.