All I know is that the stuff in roaming generally follows you around on domain logins. So if you're bouncing around on corporate computers with the same standard set of apps installed, if they save their stuff in roaming they can retrieve it anywhere you log in.

Which can be a blessing and a curse because once roaming fills up with all sorts of crap your logins on new computers take forever as the network has to transfer X gigabytes of stuff in roaming to that PC.