Let me count the merits of the humble mountain bike.
It’s good for commuting around town, fun for riding paths through the local hills, useful for exploring nearby forest service roads and exhilarating for spanning high mountain trails. It’s great for exercising both legs and lungs when fighting gravity. It’s equally good for squeezing the adrenal glands when milking gravity. It makes many dirt roads that would be uninteresting to walk worth a visit. It demands interesting and entertaining technical skills for the both the uphill and downhill. And it helps you evaluate the mettle of the people you ride with. That’s a ton of positive spinoff from one activity.