Disney calls it online check-in. Doing it is supposed to help speed up your experience at check-in, it's supposed to reduce the time you spend checking-in at the front desk on your arrival day. You get into a different line than the people who didn't use online check-in.
Some people have posted that it's been fast, and other people have had the experience that the online check-in line was longer than the regular check-in line.
If you have any special requests for your room, you mght be better off waiting to do the traditional check-in, so that you can ask to have those requests granted if they weren't already granted.