example
Go to your user control panel here: http://forum.toribash.com/usercp.php
It shows you all your subscribed forums, that way you can find them all quickly instead of having to search for 2 seconds on the forums.
Next to that, it's possible to add subscribed threads, and in the options you can set something that should give you an email if there's a new post in that thread. But I don't know whether that works.
It doesn't automatically subscribe you to all threads within the board. But it's pretty useful to put all sections of the forum that interest you in one place, so you can browse the forums quicker as zwouter said.
You can see in the control panel when you have unread posts within the board, too. But keep in mind that it also tells you there are unread posts on boards within the board you subscribed to. (which I personally think is pretty lame)
example
In this example, you can see I have checked every thread within the Rapid Threads board, while the Off-Topic boards still have unread posts in them. Basically the same throughout the entire forum.