The lastnode_id is used internally to enable something called softlinks. (It's not used externally on PerlMonks, though you can see it if you go to Everything 2.) As the other posters have posited, soft links are a navigation aid.

Linking theory states that as users request nodes from the database, they'll create interconnections. If a soft link exists, the user may follow it to the destination, strengthening it. Links that aren't followed will eventually disappear.

It's not as useful on PM as it is on E2, but it could help things out.