Have you entered anything in it? It is the default email address that is used as contact address in mails sent by the system. It will also be the sender, if nothing is specified in Mail Settings
now I remember (this setting is no longer changed in a website life !) !!!
I propose you to move this setting in proper place, e.g. in mail settings. We have several questions in support forums about contact module --> If we can easily prevent such questions...
Why not increment this field by default with admin email set @ install (if it's easy).
marco