Let's just say you want entry into a bar, and they'll only let you in if you have certain coloured stickers on your forearm. The three combinations you can gain entry with are: red and blue, purple and yellow, green and yellow stickers. If you want to get into the club with red and blue, you must not have any other colour stickers on except for red and blue. The same goes for the other two entry combinations. Instead of having to make three different links to the same page with different link restrictions, you have the one link and put these conditions for the link to work in by using $LINKSTATE. You can't have multiple different link restrictions without scripting.
That, and it's a matter of convenience in regards to seeing all variable-related events in your storygame in one place (ie. the Scripts tab in the Advanced Editor).
It would also allow you to make a script either disappear completely or just be visible but not clickable depending on different restrictions - for example, if you've never heard of the bar the link won't show, if you've heard of the bar but can't enter the link will show but will not be clickable, and if you've heard of the bar and meet the entry requirements the link will be clickable.
