The probes give the fire control computer accurate information about the flow around the weapons pods to help correct the aim point and make the weapons more accurate. This is especially valuable for the folding fin rockets, which are very sensitive to local angle of attack, and tend to point into the relative wind upon launch, confounding the gunner's best intentions. The chaiotic flow under the rotor in a hover is somewhat better understood using such probes, allowing more accurate fire while hiding in the bushes.
The Apache uses 2.75" folding fin rockets, which carry various warheads and are quite effective against soft targets. They are cheaper and more plentiful as compared to the Hellfire, which has been described as "boosting luxury cars down-range." Shots of trucks being taken out with Hellfires in Desert Strom drove commanders nuts, as the saying goes, "It would be nice if the target cost more than the bullet." FFAR's are cheap, effective and plentiful, and were the mainstay of attack helos for decades.