Rank doesn't give you much... in the sense that nobody is going to make a professionnal carrer out of ZK.
I suppose it gives you bragging rights.
Functionally, in teams, it also makes you the person to which all units/buildings are transferred to when people go AFK, resign or teams are uneven and you get an extra commander.
You can argue that caring about rank is pointless just as much as not caring about it is cynical.
Messing with ranks has the potential negative effect of making the balancer misbehave. That doesn't apply to
DocSleep because as far as I can tell they never joined public team rooms.