why do dairy cows need to be impregnanted yearly?

They produce milk when they have a calf, once the calf is weaned, the milk dries up. Even just milking them daily by machine will end up with reduced output. So the cows are put with the bull when they come into season (they start humping each other in the field, it's called 'freshening', those are pulled to the side and put with the bull).