The person who set this up would clearly know what he was doing or he wouldn't have been able to make it work.
I'm guessing it was a move in the making. The intention was probably to move over to pfsense, drop the fortigate and perhaps remove routing from the switch. It's possible the fortigate is old and can't handle routing at line speed, hence the L3 routing in the switch.
I'm guessing the fortigate and the switch was setup long before pfsense.
If the fortigate is the actual edge device, maybe this was setup as a simple way to bypass the pfsense for fussy devices or troubleshooting?
I could see setting something like this up so that you can tell a remote user "go plug it into xyz switch and let me know if it starts working"