A known (test tube) benefit of garlic is its vitamin C content.
As long as that isn't destroyed by your cooking, you'll keep
that benefit.

However, most Italians around here believe that the real health
benefits from garlic come when your whole body smells of it.
The joke way of putting is, a garlic a day keeps the doctor
away because people with colds stay clear of your smell!
You can't get much proof of that out of test-tube science, but
to the extent that the belief is true, cooking garlic cuts
your smell considerably, but not totally. A garlic (a whole
garlic, not just a clove) will let you fit in with Italians
anywhere, even if you cook it.

Garlic (and onions) can be toxic to pets.