why can units guard themselves?

a few times my crab or some other unit refuse a slight move order and i look closer and i learn they had been auto-issued a self guard order.. whats that used for?

i did do it ??? O.o ??? i guard them by mistake but its a useless command that gets in the way of fine micro

Its particularly annoying when you can't move a crabe a very small distance on a pillar due to it just changing to guard instead of move.