It was the equivalent of PowerTools(/Tweak/XP Toy? possibly had another name) for Windows XP in that it made many Gnome & Compiz tweaks one-click options, which given Ubuntu's easy-to-use focus went down well.

Things like displaying certain icons on the desktop (which I know in Windows used to require a registry setting without a tweak tool), or the contents of your home folder on your desktop.

One of things us Linux heads liked the Ubuntu of old was how customizable it was. Ubuntu Tweak allows you to tweak the OS without having a PhD in Ubuntu or the command line.

It allowed people who didn't know how to use google to tweak some aspects of their computer. Going by what I've read while attempting to help people in the Ubuntu forums, Ubuntu-tweak was roughly as reliable as pasting lines from google searches into a root prompt.