You spend a full day out shooting, filling up memory cards with photos, and then it's time to load them onto the computer and take a look. You start looking through the images, and you find yourself rotating those vertical shots — and waiting for it every time. Maybe this isn't a problem with the newer cameras, but I know that my camera doesn't auto rotate my vertical shots. In addition, I don't have the fastest computer on Earth, so it takes a few seconds to rotate each image. I don't want to deal with that! I just want to look through my photos.
To save yourself some time on the computer, rotate your images in the camera before downloading your files. The camera is typically much faster at this operation than the computer, so you're really saving a bit of time overall. Plus there are usually times between shots, or on the trip home, when you're not actually using the camera. These are good times to flip through your images, make sure you got what you wanted, and rotate the sideways ones. Now when you get home and start going through your shots, you won't get held up rotating the images.