GetIt is not an application-getter/installer. It just provides you with one single interface to take advantage of your favorite application-getters, all in one place.
When you want to install something, you shouldn't have to think "Hm, which windows apt-get clone has this program that I want to get?".
That's where GetIt comes in. It indexes repositories locally, and lets you choose which program to install from a master list of ALL available programs.
You choose the program you want to install, and GetIt knows which app-getter to talk to in order to make the installation happen.