It is also used to fill out the real name in the compose window. So every email will be like this:
To: DISPLAY NAME
Since display name is mandatory in the addressbook, this makes RC always add the display name to the email address if you use the addressbook. It shouldnt have to be mandatory in the compose window.
It isn't mandatory in the compose window, if you use an address which is not in your address book. I don't have another webmail client around, but when I want to add an entry to my groupwise address book, a Display Name is required.
And what use is First/Last name then? Just informational? Still confused
Yes, just as any additional field someone might have added. I.E. I use a LDAP addressbook and have several extra fields in it like address, city, phone number, mobile.