That's the (sad) joke. The shuttle was one element of a larger plan, where the shuttle was an enabler of the broader plan but not really a useful end in itself (vs other rockets, etc.). The rest of the plan was cancelled before being built, but the shuttle existed. There was a mostly-post-hoc justification of military/IC missions (on orbit servicing of national security assets, and "nonconsensual servicing" of foreign/hostile objects on orbit...), but this was mostly turf protection. ISS was than mostly to compete with Mir, and then became some combination of "justify the shuttle we have at home" and then "jointness missions with Russia".