One good thing is you can mousedrag a selection-rectangle over your base and give a move-order to all mobile units, without accidently moving shields that were meant to protect your base.
Static shield is taken from BA game, then a walkable shield was added, the morph gadget used to be very popular, tada there is a morph between the two. Then morphing fall out of favor, but the morphs between statics and mobiles remained. Or something like that.
Spring mods are more a guided evolution process than intelligent design out of void.
Not everything has a real 'point', what is the point of hair on your knees, some things just are.