Actually by burying the rose, what one is doing is protecting the crown from the vagaries of harsh winters. The crown is where one finds hidden the dormant cells that will sprout out new canes. The crown area is also where new root buds are kept alive. The crown is the most critical part of the rose for its survival. When one grafts a rose onto different root stock, then one moves upward the cane buds ( above the graft union) whilst the rooting buds are still kept in the rootstock crown. Of course, reducing winter damage to the root ball as a whole is something to consider too.
Burying is just one strategy to overwinter roses. Wrapping up canes with a dry leaf mulch and tarp is another. There are plenty of rose growers here that have found different suitable ways to overwinter roses, including those that lift them and bring them inside or those that grow them in large pots and move from inside/ outside during their short warm growing seasons.
In my place, because I have very mild winters, I don't really have to worry about winter damage. I still keep my bushes with the crown ( or graft union) slightly under my soil line, because my main negative threat are late spring freeze/thaw cycles, that deplete the upper canes of viable buds. Thus protecting the crown or graft union will ensure the viability of my bushes.
Arturo
