[Imp]

Gila I would guess its a last ditch attempt to stop them as infantry that close are dangerous. In an ideal world you never let them get that close but just in case they do you drill to throw everything at them including running them over. Moving makes far more sense if it happens to make boarding more difficult & if you miss them it will keep them occupied & scare the heck out of em. As to diving under the tank hope the grounds flat or theres a depression.

As to it in game as a wish why the tank already fires everything at them & as I think Mobhack said they have to do a tank check. It should be a crew decision anyway out of the hands of the player as the AI would have to be able to do it. But at this scale what diffrence would it make you might occasionally run over a few men, but on the flip side from what ginger tanker said at the risk of exposing a crew member to small arms fire.