IMO, it's usually just a quieter time to go. Most places, schools are back in session, summer vacations are over. Most Vegas hotels will usually offer cheaper rates during the week as well. Not just during this time, but all year. Anytime you can go during the week to Vegas is always better and slower.
Being the first week is also Labor Day weekend, most people will usually do a long weekend during the weekend and not during the week. Most people will get there Friday. Stay Friday, Saturday and Sunday night and leave on that Monday. Business tends to slow down for them after so to get people in, they'll offer better discounts.
Some shows may be dark (meaning they will not show on certain days of the week), but it's not usually for the whole week. Usually they might be dark for Monday and Tuesday, then show the rest of the week. Not all shows are dark at the same time though. I noticed they will try to alternate days that way there is some type of show available each night.