You can easy see what does Fortimanager does in Fortigate if you run the following command
diagnose debug cli 8
diagnose debug enable
In firmware version FMG 6.4.7 FGT 6.4.7, what it does is to get the following commands
get sys status
diag sys ha checksum autoscale-cluster
get system interface
get system interface physical
get hardware status
get mgmt-data status
diagnose test update info contract
