This is the "other" reason why eye contact is important: the players in your Dungeons and Dragons game are relying on you to deliver the critical parts of the adventure environment. The visual (non-verbal) feedback you gather from the players is crucial in ensuring you are communicating your adventure environment clearly.
If you are looking at something other than their eyes (i'm referring here to the boxed text, folks) you're probably missing some important information.
Boxed text has always been a bugaboo of mine: I don't like when it is read to me. First, I can read it faster that you can say it, so just hand me the text and i'll read the damn thing myself. Second, the delivery is often mediocre, so I tend to tune out while it's being read. Third, boxed text sometimes delivers information that is superfluous to the encounter, so you just wasted my time and yours. Fourth, the reading of prepared text seems artificial and therefore interferes with player immersion.
I disagree with those who think that information about the environment must be pried from the DM. The DM acts as your eyes, ears, and rest of your senses. Therefore, when the players first enter an important area, any information that is easily noted (particularly if it is relevant to the area or encounter) should be immediately and clearly revealed. Again, eye contact is important here: the players will give you non-verbal feedback as to which parts of your delivery were processed and which parts were not. I'm never shy to repeat something, if I don't think the players processed that information, the first time around.
It is trite to say that people only hear 20% of what they are told. This axiom applies when a great deal of (or very complicated) information is supplied. That is, the more information you communicate, in a continuous stream, the more difficult it is for the receiver to keep all of that information in the forefront of their mind, and sort out the extraneous from the important. That's another reason why I dislike boxed text. The players sit there, passively, while the DM reads, and reads, and reads, and reads. Without eye contact, you can't tell when the players are beginning to tune you out.
When drawing dungeons, outdoor environments and other adventure locations, my preference is to include little bits of one or two-word reference text, and small icons representing certain features, on the map. This allows me to quickly confirm critical details, so as to spend as little time as possible looking down at my papers, and as much time as possible looking at the players. That way, I can easily assess whether I have successfully communicated the environment, and anticipate what additional information is needed to help the players navigate the adventure.