Worldbuilding

It could also be a name for someone who originally did just guard primroses. Maybe there was a sort of tulip mania, but for primroses, which increases their value drastically - leading to some noble dickhead delegating resources to protect their flowers. At some point there's no longer a need for guarding roses, but due to aeathetics or prestige or something the guard sticks around.
Time passes and, due to the prestige associated with being the guards that dont actually have to guard stuff, the requirements to be part of the fancy guard increases. Or maybe the people in charge just realise its pretty nifty to have a guard company at hand. The guard then becomes the sort of de facto castle guard and/or then morph into some elite unit or whatever. Kind of like the Swiss Guard in the Vatican or the varangians back in the days of constantinople (which is now named Istanbul, but it was once constantinople. Nobodys business but the turks I guess.)
Time passes more, there's no longer need for a castle guard, but we're not going to disband the most prestigious military unit. Instead it gets expanded responsibility.

There's plenty examples of units with names that once made sense but no longer do. Cavalry regiments without horses, airborne units without parachutes, the marines operating in the desert far away from any shoreline.

For fiction analogues there's Sharpes' Shooters which start out as a sort of fancy skirmishing unit and end up being the 1800's Viet Cong/whatever Castros guerillas were called. Dont get excited they're British and fighting for the crown against Napoleon.

It can also have been a derisive name that the guard then took upon itself. I think the SAS did something like that because of the original uniforms looking like shit?