I'm not sure if I understood the facts correctly, but assigning climb rates is a very useful method for separating two departures (for example) with similar routes. In procedural environment. Gives lots of time to other tasks, you don't have to pump them to new levels every 20 seconds.
Preceding aircraft is cleared to, say, FL240. Let's give him 1500ft+/min until passing FL110 (rate depending on the aircraft type). Succeeding aircraft is restricted to FL100 and given 1500ft-/min until FL100. We'll only wait the first plane to pass through 1000ft AGL before issuing takeoff clearance to the second, and there it is. No problem, done that many times. Naturally this method works all the way up to the cruise levels, but here it's not necessary since they enter radar controlled ACC airspace and some other separation can be issued.