Lynata said:
Another matter worth remembering is that even the transports used by the Imperial Guard give over a lot of room to allow for training and drilling while in transit - vaulted cargo bays repurposed with fake warzones are a real possibility. Consider, by comparison, the amount of space you can give over to all manner of useful support roles in a Strike Cruiser… give each squad their own quarters arming chamber, meditation chambers, chapel, training facilities and more besides… a munitions forge to allow the manufacture of ammunition and repair of weapons, armour and of vehicles, an extensive apothecarion to treat battlefield injuries and safely store recovered geneseed, a company temple and reliquary, an extensive librarium and fully-equipped strategium, all with assorted complements of serfs and servitors to serve those functions in addition to those crewing the ship…
…in essence, everything that an Astartes Battle Company would logically need for independent operation for months or years (or maybe more) without returning to home - vital for Chapters operating in multiple theatres across the galaxy. And Battlebarges would be moreso… a literal home away from home.