Comment 1 for bug 379893

This is not the only reason. From bug 414627 (marked as a dup of this one):

"This would be a nice feature especially when mentoring a new bug triager. It should be similar to bugzilla where you get a copy of the person's E-Mails from the system. Obviously a user should only get one copy of a bug report.

Would also be nice to get a header telling you which user you are shadowing to help with filtering bug e-mail. Another feature that could help with the filtering would be to be able to specify an alternate e-mail address. For example, if my gmail account is <email address hidden>, I might want the e-mail directed to <email address hidden>."