I can understand having a favorite forum, but why a favorite topic? I've never seen a topic last long enough to be a 'favorite', and even the greatest topics fall.
Whats the point of a favorite topic that last 2-3 weeks at most?

Keeps it at the very top of the forum for you, and you can easily change them out as you wish.

I find it practical to keep track of various threads that tend to pop up once in a while. I don't use favourite forum though, as I generally only use the CD, and own so many games from the catalogue anyway

lets you keep the dvd images thread pinned.

There's a bunch of threads that are actually useful and keep getting updated, like the gaming deals one, the DVD covers one, the demos one and so on and so forth. unchecking them is not much of a hassle, so yes, for people who don't like to wade through pages to find old threads, making them a favorite is easier than searching for them.

It's like a personal sticky. For example, I'm a Linux user so I have the Linux compatibility threads checked as favorite topics.

I have things like the last game/film you played/saw as favourite, because they're the ones that will stick around a bit, like the What is this game? thread.
I'll usually do it for my own threads too, although I haven't made a thread in ages.

Demos links thread...enough said. =)