Terraforming serves the same purpose in KSR's books as it would in real life, in that it fulfills the dream of having a second Earth. A family or group can go out into the Martian landscape, find a pretty spot, and set up a homestead there with a minimal amount of skills, technology, or outside help (beyond what is needed for agriculture). That angle was a big part of Green Mars - I haven't actually read the other books in the series and I hear they differ quite a bit in focus. You can see the dream of terraforming Mars as a petty or vain thing, and I won't argue with that. But take a step back and look at the reasons people give for why we need any manned spaceflight program. It's always something like, "We need to colonize space for the future of the species." Putting humans in space is, at its core, about propagating the human experience out as far as possible. Terraforming works to that end. The goal is to make a new home, and people will prefer it to be as comfortable as the old one, and as similar as possible.
Speaking practically, any real Mars program will start small with the people living in a pressurized, heated chamber. Either that, or you'd need to start the terraforming a century before anyone arrives. Another thing to consider, since we are discussing KSR's books, is the safety aspect. Killing off an entire colony is as easy as ripping a hole in the tent covering it, or attacking the equivalent weak point of an underground structure. When you're dealing with humans, conflict is always a possibility. Especially as the colonization advances and you have thousands of people aligned with different groups. Having a breathable atmosphere is a godsend when war turns you into a refugee, or your colony has some technical or natural disaster that puts it out of commission.