IFVs have always been slightly armored boxes to transport infantry but usually they were at the mercy whoever was driving the thing and if there was somebody with the nerve to man the machine gun out in the breeze. The BMP not only had a bad-ass turret but firing ports for the passengers.
Firing ports are pretty much useless. If the vehicle is moving, hand held weapons won’t be accurate. If the vehicle is stationary, it is a conspicuous target that doesn’t have enough armor to protect the people inside.