It's always a pain to remember the email address of each contributor by heart (I often try the IRC nick, but it's not always part of the email address). With the ajax_user_autocompletion parameter turned on, when you start typing an email address e.g. in the CC field of a bug, Bugzilla displays a list of matching addresses in a drop-down menu. And if you don't remember the address email itself but only the name of the user, Bugzilla also looks in the real name of the user so this makes it much easier to find someone.

There is no risk that someone tries to use this feature to steal email addresses for spam, because 1) you must be logged-in, else the feature is not enabled, and 2) the list is dynamic.