For me it was that I wanted full-screen Terminal without a scrollbar. Yet I like having a scrollbar in my browser (which indicates page length for instance) and the scroll visibility is a systemwide setting in OS X. (Actually there is a plist-app-specific version of the setting which Terminal.app erroneously ignores and Apple refuses to fix it). With iTerm2 I get a scrollbar in my browser, and a clean black full screen background in my terminal.