This isn't something newer BMW owners have to worry about, but most cars still have spare tires. Many of them are just donuts, but on some expensive cars they may be a full size aluminum wheel and full size tire.
4 of the same car = one full set of brand new wheels and tires.
Off to eBay they go and your out a nice chunk of $$$ that was sitting in your trunk. Not to mention the inconvenience when you actually get a flat.