Trees begin to heal the wound made by pruning by attempting to wall off the wound with a collar like growth. Each tree branch has a collar around the branch where it attachés to the trunk. You may have to look closely for relatively young branches, but the collar is there.  Cutting the branch as close as you can to the trunk will destroy the collar and the tree will be unable to wall off decay in the exposed wood.

The best way to remove a branch is to make a cut that follows the collar around the branch. After you have identified the collar, begin your cut just outside of the collar. The branch collar will then begin to swell and grow so the wound will heal over and prevent decay. I have a silver maple that suffered pretty extensive storm damage about 10 years ago. A large branch, at least 12 inches in diameter was broken so we cut it off. Now the pruning wound is completely walled over the cut and there does not appear to be any decay in that part of the tree. Evidently we did not cut cut too closely and our tree is still growing nicely.