Practically it is a desktop variant of our own Installer.app. It will allow you to view and install packages that are present in various Installer and Cydia repositories by downloading them to your computer and then synchronizing via the USB cable with your iPhone.
With InstallerApp you will be able to install many of the available third-party software packages, both requiring the system partition to be writable, and the ones that do not. Yes, there are less packages that don't require writable system partition, but it does keeps your iPhone more secure.
Once installed on the iPhone, the package will be stored on your computer's hard drive and InstallerApp will (just like iTunes) notify you of updates. You will also be able to quickly reinstall the packages you have after the restore, without having to select each of them individually.
We got plenty of plans on enhancing this solution - for starters lets mention that in the future you will be able to get trial versions of the installable commercial software... and much more.
InstallerApp is in beta testing now and we anticipate the 1.0 release soon. We'll keep you posted on the progress.