Besides the portability of, well, portable apps, there is also privacy which I think everybody likes. There are utilities for antivirus etc., but I have yet to find one that cleans up after you work on a computer.
The ram footprint it has is extremely small and it's installation folder size is only up to 2MB. If it was ported to flash drives, I think it would make a great app.
-- CCleaner (http://www.ccleaner.com/)