When packing for a trip or loading up get out of dodge gear, space is almost always at a premium, and clothes can be a space hog. Sure, you can cram them down as small as possible, jump on the bag, etc. but then you get a winkled mess. Many have discovered that tightly rolling clothes saves a lot of space over traditional folding. As a plus, rolled clothes tend to come out a lot less wrinkly than folded ones, even when compressed.
Guys in the military have developed a pretty cool technique for rolling t-shirts. Here's a YouTube video that demonstrates the process