It was designed to support a specific Air Force requirement: the ability to launch, release or capture a spy satellite, then return to (approximately) the same launch site, all on a single orbit. (I say 'approximately' because a West Coast launch would have been from Vandenberg Air Force Base, returning to Edwards Air Force Base.)

The cargo bay was sized for military spy satellites (imaging intelligence) such as the KH-11 series, which may have influenced the design of the Hubble Space Telescope. Everything else led on from that.

Without those military requirements, Shuttle would probably never have got funded.

I'm listening to "16 Sunsets", a podcast about Shuttle from the team that made the BBC World Service's "13 Minutes To The Moon" series. (At one point this was slated to be Season 3, but the BBC dropped out.) https://shows.acast.com/16-sunsets/episodes/the-dreamers covers some of the military interaction and funding issues.