Your ears play an important role in making you a better official. When roving, you often hear a variety of noises on courts behind you. Even subtle things such as “are you sure” (more than once), excessive racquet hitting the tape/floor/curtain, loud comments of disgust directed at the player him/herself, etc. can mean there are potential problems and you should move to the court in question to observe.(This is particularly helpful if you are covering multiple courts) Obviously, there are the blatant, code-able offences such as obscenity/racket abuse. A new rule for 2011 states that “roving umpires may impose a penalty only if they have seen, heard, or otherwise observed enough to determine with reasonable certainty that a violation has occurred.” (2011 FAC, IV.D.6c) In other words, although you no longer have to be in direct observation, you must be absolutely sure of who/what you’ve heard (you can ask the player) in order to give a code violation. (Interesting that there was a question on the most recent USTA exam concerning this rule.)
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Pacific Northwest Tennis Officials Association 
Well put Betty. Quasi-Omnipresence (not omnipotent).
When roving, an official’s ears almost become part of their peripheral “vision” — or at least they should. It’s interesting to note that when you’re officiating a match from the umpire’s chair your sole focus becomes the match in front of you and you can turn off (or at least turn way down) the attention you pay to what is happening on courts that are not your responsibility. Of course there are other layers of your officiating “senses” that get turned on when you’re in the chair. You must announce the score, mark your scorecard, leave the chair at the set breaks to measure the net (and, if necessary, put new balls into play), etc. Your eyes and ears get to stay focused on just one court, but that focus must be more intense than a rover’s focus can be — every ball, every call, every question is your responsibility. You cannot afford to be distracted by what’s happening on another court.