Winter tyres

I need to get winter tyres for my 2017 Audi Q7 TDI 272 Quattro S line. It came with 21inch summer tyres as standard. My local Audi garage has said they will only supply and fit both the wheel and the tyre and it’s a smaller tyre and v expensive. The parts man wasn’t too forthcoming with info. Does anyone know why I need to buy both the wheel and the tyre rather than just the tyre. I’m out and about in the alps a lot and need a v good winter tyre so will spend if its a safety thing but I’ve never heard of having to buy new wheels as well. I didn’t have to do that for my freelander. Sorry if this is stupid question but I’m new to Audi’s and also don’t know much about cars!