I never use Launchpad, but I'm glad it's there since I've always had trouble explaining the Applications folder vs the Dock to people, and Launchpad solves that very nicely for non-technical people (especially ones with iPhones/iPads).

Nope, and you don't have to. But for those users that are new to the platform (and I have watched quite a few) it makes them feel at home. They instantly know where to look for their apps, they can re-arrange them however they like. It is not for those of us who grew up on Mac OS, then Mac OS X, Linux and Windows, it is for those that have had an iPad or an iPhone and know there is a single location (home) where all your apps are. Launchpad fits that bill nicely.

I use it. Sometimes I actually forgot what apps I have installed or forget the names of them if I don't use them frequently. If my hand is already on the trackpad I just have to pinch in and there they are. Quicker than going to the Applications folder.

I've been using OS X for years as my main OS, and I do occasionally use launchpad. I've got a boatload of stuff installed and I don't always remember what I have. Launchpad provides a nice way of viewing all the apps.

I don't use it everytime I launch somthing (I use alfred for that), but to refresh my memory every now and then.

It doesn't work exactly like I would like it too, but it's not bad, and it's very out of the way if you don't like it.