Bastion Host

I have been reading up about firewalls and had some questions. If ports 25 and 80 are being forward through to the accepting server on the inside (Xchange & WWW) what is the point of a Bastion. Forward 25 & 80 to the second firewall then that firewall will forward the ports through to the servers. How would this be an enhacement to the firewall protection?

Is there maybe a different way to do this.