ATGM = SAM

[PlasmaKrab]

I don't see Tows or Hellfires turning fast enough to even keep up with a helo unless he's in a hover or really slow.

That is precisely the point. AFVs fire their main guns and ATGMs on low-alt, hovering helos only (mainly).
Some modern ATGMs are meant to be fired at helos when need arises (BILL, Javelin...), and if you ask TOW gunners, they should have been trained to fire at atack helos in self-defence.