I'm no tyre expert but think the benefits of a wheel/tyre set vs. just a tyre swap is:
- The action of removing/re-fitting tyres is not good for the tyre.
- You protect your summer alloys from salt and corrosion by using a winter set.
- You can opt for a slightly narrower setup for winter (e.g. 18" vs. 19" or 17" vs. 18") as this gives better performance in ice & snow.