D&D is a came of the mind. It requires a vivid imagination, and the ability to communicate your ideas with the others at your table… Sometimes however your ideas don’t translate well without a visual. Building your own terrain, drawing out a map on a mat, or using tiles from something like Heroscape. (Seriously Mike Mearls, you and Perkins told me a year ago that there was something coming, and I’ll be asking you about it again at PAX South and East this year)
As well as you describe something, as attentive as your players are, sometimes you just need a visual. Maybe this combat has areas that can be used by you or by the party for a tactical advantage, maybe there are hazards in this room that only a visual will help with. Visuals can play a big part in your sessions.