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.
When working on large images I am often zoomed in to 400% and only viewing a small portion of the image. This makes it difficult to see any changes in context to the entire image. You could of course zoom in and out to see the overall effect of your changes to the image (the novelty of doing this will soon wear off).
The easy answer is to use the View > New View menu and create yourself a new view of the current image. You can set different parameters for your new view such as the Zoom.
I had a recent requirement to add multiple copies of the same image to a larger sized image for printing. Effectively I needed to tile the background of the larger image with my smaller image without any gaps. As there were many copies of the smaller image to be aligned I didn't really feel like trying to manually align each of the smaller images (I have better ways of spending my time).