We're just used to assuming people are not familiar with partitioning and other more indepth things.
I've never played with Slack myself but from what i hear the install is pretty good. It just cant resize NTFS partitions yet. For that you'll need a third party app or something windows. I'm sure you have that.
I just looked up the slack forum here and it looks like this could help some since you now have an idea of the partitions you'll need.
You'll need minimum a / (which is root) and a /swap
The extra partitions are for extra security and data safety. One partiton can go down and you can make a new one to replace it or just reformat it and reinstall just that partitions stuff. Yes you can totaly wipe out the whole OS and just by reinstalling it you'll have almost everything back the way you had it before.
I don't know if slack needs the /root partiton but I know mandrake doesn't. It just makes it a directory inside the / partition.
Each distro has a slightly different demand for the partitions and sizes and of cource what you install determines the size needed also. But in the end they are close to each other.