Well, are we talking Starfleet rear admiral (which, depending what you read has two halves.)
In
naval history, a rear admiral was an assistant to a vice admiral, and commanded the rear section of a squadron of ships.
In Star Trek, the
Rear Admirals we saw were often Academy commandants, or in the equivalent role of commodore, when that rank was discarded, hence the upper and lower half.
As for the rank system, I would be curious to hear recommendations for a different type of system. Really, the Starfleet one strikes me as very functional but not always employed consistently, due to the needs of the story.
And I'm glad
The Wormhole pointed it out the enlisted ratings often are far more experienced and knowledgeable than officers, especially junior officers. Having known several enlisted servicemen, they have put in many, many years in to their job. Officers are not always as experienced, and often have to oversee many other tasks.
A good, new, officer listens to their senior chief.