It's pretty much as Stewart45 said, because you drop the liner down from the chimney, if it hasn't been swept, then it'll bring all the soot down with it. I got the chimney swept first, and the guy cleaned out a flexi bucket full of soot. If I hadn't swept it first, then the chimney liner would have brought that lot down on the person pulling the liner down.
Chimney tend to get swept once a year, but if you're putting in a liner, then it's best to surround it with vermiculite, and that'll stop it banging around whilst it's being swept. The drawback to the vermiculite, is that it stops the upstairs rooms benefitting from the transferred warmth.
Come back to us as you do the job, and we'll see what we can do.