Just what were Guardians supposed to be anyway? Well, some sort of quasi-military peacekeeping force, obviously, but what was their purpose or mandate? The early seasons make it seem as though only Guardians were capable of defeating the User in Games, but as we later found out when Bob left, Dot could handle a Game just fine without Bob, and with one exception, Enzo and Andraia did just fine on their own as well. The credits monologue says Guardians "mend and defend" though Mainframe had its own Binome defense force that seems to serve a combination of military and police department
Guardians mend and defend the system itself. I figure they're sort of like antivirus and damage repair programs. The binomes handle everyday crimes and crises that befall the inhabitants of Mainframe, but Bob defends against attacks on the underlying integrity of the world itself, on its programming and operation. Megabyte and Hexadecimal, the main villains, were both computer viruses trying to infect Mainframe, and Bob's job was to protect it from them. The games were a different type of intrusion, though one that could be just as destructive to the individual sprites or binomes trapped within a game, since they'd be turned into Nulls if they lost.