You can read about Service routes from PAN directly here.
Basically … “The firewall uses the management (MGT) interface by default to access external services, such as DNS servers, external authentication servers, Palo Alto Networks services such as software, URL updates, licenses and AutoFocus. An alternative to using the MGT interface is to configure a data port (a regular interface) to access these services. The path from the interface to the service on a server is known as a service route. The service packets exit the firewall on the port assigned for the external service and the server sends its response to the configured source interface and source IP address.”
This is generally used if you configure the firewall, but don’t actually happen to physically plug anything into the MGMT port of the Firewall (MGMT on Physical or VNIC0 on VMs). However the device does have a internet connection, or has some interface on the dataplane that has access to a specific service. Whatever the need may be they can be useful to know they exist and can be utilized for certain situations.
When I discussed this with a friend who deploys many of these devices, it was opted to use the MGMT interface for most things. I did note one case such as Email, where you could configure the service route for that via the gateway interface for the mail server, thus only require one IP in the ACLs of the mail relay/server.
He did note that you could not test email from the passive firewall, as the interface won’t be active. Which could be problematic for other monitoring services such as SNMP, if utilized. Which was noted. Luckily many different services (SNMP/Email/LDAP) can be configured independently and all default to the MGMT interface.
The main reason I even noticed this was due to email not working on the alternative firewall after it took over from a failover, even though the dashboard on both firewall stated the running configs are both the same. Well it turns out that service routes I guess are not tested for synchronization between peers.
So yeah… not that if you are using Service Routes with PAN firewalls.