Why should the carpet be vacuumed prior to professional
If you have your carpet cleaned professionally, you will not have your carpet vacuumed unless you vacuum yourself.
The need for dry vacuuming is well documented and recognized in various industry guidelines including the IICRC carpet cleaning standard S100. The reason vacuuming the carpet before cleaning is one that has been proven as well as making good common sense. Because most cleaning methods use water at some point to remove suspended soils, it is recommended as much dry soil should be removed as possible before introduction of moisture.
Dry soils include: sand (and other dry soils like clay and feldspar), human and pet hair and skin, and dust. These types of soil are best removed when dry. The correct type of vacuum - one that has a separate motor for the beater bar and the vacuum bag - will separate the carpet fibers and remove these soil.
The beater bar is key here, because the steam cleaner wand or other cleaning device is not designed to separate fibers to remove dry soil. The steam cleaner wand has a slot that glides over the carpet spraying a mist of (hopefully) hot water followed by a vacuum slot. Typically the cleaner applies a water based traffic lane cleaner before this step, so if there is no dry vacuuming before this step, you can pretty much count on the fact you are getting a substandard (remember IICRC S100) cleaning job.
The typical carpet cleaning machine (no matter how large they are) is in reality an enlarged canister vacuum with a water supply. Once this canister vacuum squirts water on the carpet, the ability to remove dry soils is compromised. As I mentioned before this is as much a commonsense argument as it is scientific, and if you think about it, when you get hair wet, it will adhere to another fiber more readily.
If you ask a carpet cleaning technician, they will give you a party line 'our units are so powerful that we don't need to vacuum'. This is repeated many time each day to customers that want to know - deservedly - shouldn't their carpet be vacuumed first. Trust your instincts on this.