Why do I have to enter an email address?
This may be because the member you are sending the invitation to has activated his or her 'Invitation Settings' privacy feature. This setting requires that invitations only come from people who know the user's email address or appear in the user's "Imported Contacts" list. This helps to prevent users from receiving unwanted invitations from people they don't know.
Members may also be required to enter an email address because some past recipients of your invitations have indicated they did not know you. In this case it helps to add a personal note to future invitations reminding them how you may know them and why you would like to connect.