Aligning oneself to a stronger, older, better country. If people will be better off with the support and guidance of and older and wiser country then so be it. Also, being aligned with someone assures that you with have no serious conflict that can lead to an unwinnable war on the weaker side.
Since the current Canadian monarch, from 6 February 1952, is Queen Elizabeth II, Canadian allegiance to the British Crown does not seem totally out of context. However the second half of the question as to whether this is good for the citizens of Canada is another matter and would depend on the social and political stance of each individual Canadian.
While I can see why many people would balk at the idea of the Canadian people feeling an allegiance to the British crown it nevertheless applies a unique character to the country, often overshadowed by its neighbour to the South, and the crown itself generates a good amount of tourism in both directions.
Canada's allegiance to the British Crown is not beneficial to her citizens. Canada is a sovereign nation with her own interests, which should guide policy. The oath is a promise of fealty to the monarch. It is an outdated concept, with its roots going back to 1689, formally established in 1867..