When you have a group of warships going to the fight, the game auto-assigns you them if you have too many. Which, yes, is nice, but if I want to grab a bunch of Space Marines for a boarding action-heavy group, having just one CL is NOT a good time.
So a way to go "Okay, give me THIS from my Space Marine fleet, THIS from my Imperial Navy fleet, and THIS from the AdMech" would be very nice, as it fits as "Okay, I'm the Admiral, YOU go in first".
Technical feasibility unknown, but it makes lore-sense and would be a nice mechanic.