Give that AC a cookie.
We need a place to house the people and tools that will assemble, test and prepare a large number of independently launched modules into an interplanetary space craft. We need a place to house the people and tools that will assemble an array of large optical reflectors into a big, orbiting interferometer with two orders of magnitude better resolution than Hubble.
There is an obvious and crucial purpose for IIS when you consider in-orbit assembly and service work. Getting to Mars etc. is relatively easy if you can incrementally assemble and fuel an arbitrarily large space craft in orbit. IIS has all the habitat, power and communications facilities to host the necessary construction crew and equipment.
IIS is a STATION. It is not an end in itself anymore than a gas station has a purpose without cars.