Ymir is the cyclical aspect of nature, consuming/destroying that which he created previously. Like a man walking around a roundabout casting newness out of his right hand and destroying/consuming that which he already created with his left.
Killing Ymir is like breaking the cyclical aspect of nature, it means that you can have forward motion and are not doomed to repeat the same cycle over and over again. It's like being given a road off of a roundabout that allows you to go somewhere else: killing Ymir allows you to do something new.
Killing Ymir allows you to construct greenhouses in
and grow foodstuffs in it all throughout the year, instead of being bound to the cycle of the weather (which would otherwise mean a shorter growth period for your crops). Iceland