Where i come from, bullet proof vests are used to stop rounds aimed intentionally at one's center of mass; hopefully, the rating of the vest will exceed the strength of the round employed.

i believe in most departments these days, bullet proof vests are worn routinely when on patrol.

They don't always work. Just before I retired, I donated two weeks leave to the widow of a ranger who caught an AK-47 round that sneaked in between the seams of his vest.