Yeah i was wondering that too. I have a theory; its either just to fine tune the optic where to concentrate, or its to show the missile the targets direction of movement. I havent tried it yet. But I have deducted that newer versions of AGM-65's IR optics follow the target whereas the B variant just heads towards the area it has been designated to fly towards and on the way picks the target in its opticview. So it would make sense for the missile to have a need to know where to expect the target to being heading towards. But i really doubt that its the latter option.
