Is there any medical reason for a dentist to give novocain (procaine) shots prior to general anesthesia?

If you are having . General anethesia, the dentist should wait until you are asleep before he injects.
Yes. The local anesthetic will anesthetize the area being worked on and will also decrease the amount of bleeding in the surgical field. Although you may be "asleep" during the procedure, it is a very light sleep and without the anesthetic your body will involuntarily react to the pain stimulus.