This is because a typical pet dental includes full oral exam (i.e. checking for oral growths or ulcers), treatment/removal of diseased teeth, removal of plaque and tartar, and finally polishing the teeth. Animals, no matter how well-trained they may be, do not open up their mouth and put up with the noise and possible discomfort involved with a full dental cleaning.Not only is the bad breath a problem, but an infected mouth (teeth, gums) can result in other, 'larger' problems, such as tooth loss, bone infection, as well as heart and liver infection from the overload of bacteria in the infected tooth socket and gums.
Most veterinarians recommend a pre-anesthesia blood panel to make sure there are no underlying health issues minimize risk of anesthetic problems. If not, ask about making bloodwork part of your pet's pre-dental work up. In some cases, antibiotics are given a few days prior to a dental to minimize the swelling and tissue inflammation during the dental work and to reduce the risk of serious infection.
Depending on what is done during the dental (i.e. extractions or tissue biopsy), your vet may send home pain medications in addition to post-dental antibiotics. If you have concerns about your pet's comfort level or post-dental care at home, please speak to your veterinarian.