Q: Why can you add email to your message options

Gives you the option of having people address iMessages to your email address rather than just to your phone number.  If, for example, you have an iPad, which can only receive iMessages at an email address, and you want to be able to see the same message on you iPhone too, you can add the same Receive At email address to both.

Messages are not routed to your email inbox if that's what you mean.  But if someone sends a message to your and addresses the message to the email address shown in your Receive At setting, the message should be delivered to you in the Messages app (like messages addressed to your phone number).  Are you saying that isn't happening?

You're welcome.