When you are out and about you may not always have your computer or laptop available with all of your required applications. GIMP Portable lets you you use the GIMP software on any PC without having to install it first.
Perhaps you are at work and you need to do a quick edit on an image. It is likely that you will not be allowed to install unauthorised software on work computers.
GIMP Portable is one of the Portable Apps which can be easily run from any portable device such as a USB flash drive, iPod, memory card or portable hard drive. it leaves no personal information behind on the machine you run it on, so you can take GIMP Portable with you wherever you go.
"The GIMP (GNU Image Manipulation Program) is a freely distributed program for such tasks as photo retouching, image composition and image authoring..."