I have seen this on bigger boards, was wondering if this is hard or not.

You can click on the user name (using "Online now" addon), which takes you to the 'About user' profile window. There you can see in which forum that user is and what topic he is currently reading.

Is this possible?

Kind regards,
Marius

I've seen it too -- but I've never got the idea why and who would need this information...?

but I've never got the idea why and who would need this information
The same for me.

It's not possible with in current default version. Every time user watches the topic you need to heavy update database with current forum/topic ID, and a lot of updating usually will dramatically affect the overall performance.

But it probably could be done in general. For a fee of course ;-)