Face it, many people don't have your favourite open source programs installed. It's a pain to see that your school computer has the entire Creative Suite Master Collection, but not a single copy of GIMP or Scribus. Maybe you're at a friend's house and need to print a photo in GIMP's .xcf format, but they don't have the program installed. Let's just say that in this case you aren't able (or allowed) to install a copy on the spot.
I've been using portable programs for many years now, and I thought that it would be a good time to share this with you. Basically, these apps do not require installation and directly run off a USB drive. You simply click on the executable and the program starts instantly, loading itself into the RAM without writing any registry keys or traces. When you close the program, it disappears completely from the host computer. It's not in any way permanent or damaging.