If you levy the army of a city state, you can disband it all when you don't want it anymore (you don't get gold for that, sorry), then they will build new up to date units. This could potentially make levying a viable strategy, especially for Sumeria.
Example: I levied Carthage army, which was enough of warriors+pults to zerg rush a 5 city emperor AI in the early medieval era. At the end I disbanded all warriors and now Carthage spammed a carpet of Crossbows, which would have been a very valuable asset in the next war, if there had been any more AI to invade at that time.
Levying to disband can also in some situations be useful just to get rid of those units blocking your way. Had to do this to get my late game Great Scientist to the preferred 6 rainforest location.