I think unless you are excellent in the art of slapping up a mic in front of an instrument and getting the *best* sound right off the bat, utilizing a control room allows you to tinker with your sound and hear a 'closer to end result', as you make adjustments, either with mic choice, placement...or something you do electronically in the control room. You are able to listen over the control room speakers VS guessing how it might sound over the speakers when you listen to a playback. That is one of the major benefits. (besides being able to say how much that last "take" sucked, and not letting the *artist* hear your comments!!)
Personally, unless you are highly experienced, getting a good sound without monitoring via control room speakers is a tough act. Out in the "studio", headphones don't give enough of an accurate picture, and more so aren't capable of keeping out the outside sounds which will effect what you are hearing in the cans. Usually with that route, it usually goes like; record, listen, adjust, record, listen, adjust, record, listen, adjust....etc. etc...
so...next question...how do you record your
acoustic guitar by yourself, and adjust the sound while playing??? record, listen, adjust, record, listen, adjust...........!!!