Hi Arix,
The source-address is mandatory when you send the Syslog out of the SRX. So, whenever you're using the line "set security log stream S1 host <ip-address>" you need to specify source-address. It is used to indicate from which source, the syslogs are generated.
If you're saving stream logs within the device, source-address is not required.
So, if you're using source-address in your security log configuration, the Syslog server will receive the packet with the source as the IP address which was specified in the configuration.