Some thoughts regarding space stations and manned missions to Moon and Mars:
While early concepts had seen the primary purpose for space stations to serve as a gateway to the Moon and other planets, this has never actually materialized in practice. No space station ever built (Skylab, Salyut 1 through 7, Mir, ISS) had a interplanetary gateway function. They were space laboratories in low earth orbit.
On the other hand, the Apollo moon landings were achieved without having any space station to serve as a "gateway". A manned Mars mission could (and probably will) be conducted without any direct involvement of a space station too. So the presumed connection between space stations and manned missions beyond LEO by von Braun and the likes simply didn't hold water in reality.
That being said, there are two possible applications of ISS program experience for future "deep space" missions. First of all, a manned Mars ship will probably be put together in Earth orbit and "launched" from there. Through building the ISS we know how something like this is done exactly. Second, the ISS helps in gaining experience with long-term space habitation and what effects it has on the human body and mind (although Mir had already served in that function as well).