The functionality is useful because it allows you to see other attributes of a user while you're searching in the reference field etc. You can read how the functionality works on the wiki here.
Basically the following line can be added to the dictionary attributes of a field (for a sys_user reference):
ref_auto_completer=AJAXTableCompleter,ref_ac_columns=user_name;email,ref_ac_columns_search=true
This will give you the user_name field and the email field (in that order) displayed after the display field. The second attribute (ref_ac_columns) lets you specify if these attributes will be used in the search as well (in this case "true").