Although coobeastie's explanation is certainly part of the purpose, it seems to have had other covert purposes such as being an accomplishment Publius Aelius Traianus Hadrianus (Hadrian) could boast about when he returned to Rome. (He, upon return to Rome, became Emperor). Additionally, it awed the local's concerning the power of Rome. Finally, it certainly did have the ulterior purpose of keeping thousand's of Roman solidiers busy. It's not good to have armed, fighting men just sitting around, so the physical labor also served a purpose.