Why would a dds give epinepherine for a filling?

Less anesthetic. The amount of Epinephrine that dental anesthetic carpules contain is very minimal. Generally 1:100, 000 or 1:200, 000. Epinephrine acts as a vasoconstrictor & helps localize the anesthetic to the area that the dentist is working on, thus minimizing the amount of anesthetic that is generally used, which is usually safer because less anesthetic is used.
Vasoconstrictor. Clamps down on vessels and may reduce bleeding.
Vasoconstrictor. Most local anesthetics in dentistry contain Epinephrine as a vasoconstrictor. That means it constricts blood vessels in the area. "so what? " you might say. The blood vessels are what "carry away" the local anesthetic making it wear off faster. If we constrict the blood vessels, that means more local anesthetic stays in the area for a longer time...This means less shots for you.