It isn't other than he is able to accomplish more with less talented players by using it. The concept of contain the spread O's while stopping the run games is what drives the 3-3-5. The lack of quality D-linemen is why teams have gone to it. They will let you run the ball within reason because O-schemes rarely will stick with the run for entire drives. The d-linemen have one gap responsibility, but they line up without revealing it by going head on against the assigned O-lineman.
JMO and observation, the primary difference with his is the big LB's he uses to stop the run games between the tackles. Pruitt, Bama and others using it tend to use Henry T sized (as a sophomore) LB's because they had more talented and physically imposing D-lineman. They also use many D schemes because they have more talent and deeper rosters. In theory, if you are going high octane offense and contain style D because of less roster talent on D, you can have more success against better teams. Hence, Leach hiring Arnett.
Good article on it below going back to Babers and Syracuse.
Syracuse’s new 3-3-5 defense, explained