Marks answer is the most important reason, no port opened from DMZ -> Backend means no attack footprint.
To complement Marks answer
- when using ML servers without relay server , sticky sessions are needed on the LB. With relay server, no sticky sessions are required since the relay servers take care of directing the session to the right ML server.
- A single relay server farm can take care of multiple ML server farms (different apps), afaria servers , SMP servers or a combination of those without requiring different ports to be opened in the outer firewall for each individual application. The different applications are routed by relay server to the right backend system.
Diether