Mainly to join trackers, because its very hard to find open registration.

To give away invites, learn about different trackers, and hopefully get access to trackers I don't have that I need for specific content.

More than 3 reasons, but on a 2nd thought it's more about the community and keeping updated with what's going on in the private trackers world.

because its right up my street

Point 4.

To find info about different trackers, read reviews, see when registrations open, etc
But now... on top of everything else, I'd say because TI is simply awesome!

To join some local trackers, it gets harder and harder to have access to local content nowadays!

If I'm going to be honest,I joined to get invites to private trackers that have British torrents

To find free invites

To get an invite for my media server.