Space is pretty big and it is in much of need of policing as the rest of the Empire.
Not only that but someone has to ferry the grunts around between missions!
This sorry responsibility falls upon the Empress Fleet. Selected from the best graduates in mathematical and physical sciences from various higher education establishments Fleet recruits are the best and the stated aim of the institution is to make the best better.
They do have a funny way of doing it
For most Fleet Academy graduates the majority of their early careers will involve serving drinks to lecherous senior officers in a uniform that doesnt exactly help matters.
Those few who get past Stewardess will have to fight tooth and nail to become Captain of their own ship.
Of course its not just Officers in the Fleet. There are also the Marines and Fighter pilots recruited from particularly raucous hockey societies. They act more like the dumb grunts of the Guardians but theyre unlikely to be a match for them physically.
Fleet do get involved in personal combat a lot more often than one would think. As well as boarding actions and the occasional planet fall Fleet are constantly in conflict with their rivals in the Guardians.
As a Warband (or away team) they bring considerable prowess in close range engagements and can dig in to a position like only a bunch of girls used to confined spaces can!