The mailing list is a place for developers to subscribe to so they can get notification updates on bugs and provide feedback to the bug in the bugtracker if need be.
The bugtracker itself used to send mail to one person, who may or may not respond, the mailing list instead allows bugzilla to send an email to the list and any subscribed users will get an email/rss update from the mailing list software regarding the bug.
Developers can then look up the bug and assign it to themselves, make comments on it etc.