There are several reasons for needing to push back the snow beyond the edge of the road each storm; all of which are with safety and cost effectiveness in mind:
1- To ensure there is room for more snow later in the year
2- To keep the road as wide as it should be for the safe passage of vehicles; especially emergency vehicle traffic
3- When the snow banks are completely off the road, warm day snow melt has the best chance of staying off the roadway and not causing slippery conditions in a sub-freezing overnight resulting in additional cost in machines and man power to handle.
4- Snow & Ice are just other forms of water- as such, they should be kept off of and away from the road as much as possible to keep this eventual moisture out of the structure of the roadway.