Maximum cable length for USB2 is specified at 5 meters, so at 15 feet you would close to the limit per spec. However you should just try it and see how you go.
Reasons for using a server are various: you don't have to open your laptop / keep it on to play music (use phone or tablet for control), you need more storage space, you like to have redundant copies for backup, you like a particular piece/set of hardware/software that is server-oriented.