Great question-- I've used the car analogy before with non-bike people, to explain the tendency to have multiple bikes.

My Trek 7.3 fx, with rack and panniers and the ability to carry a lot of crap, is like my nearly 10 yr old Subaru Outback. Sturdy, reliable, practical, but kinda boring. The other bike is a 1992 Cannondale R400, and I think of it as an older convertible, horribly out-of-date mechanically and not very good in bad weather, but fun and fast.