There are many Windows implementations of a Linux style apt-get function that let you type "install Firefox", and poof, you have Firefox installed. 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.
While GetIt comes with its own GUI to let you pick which applications to install, the only way to be able to incrementally search through installable programs is to use GetIt with Find And Run Robot.
You can now use the GUI to chain install as many applications in one go as you desire. Great for setting up new PCs with the latest versions of your favorite software.