and welcome to the forums.
Some FGTs have a switch circuit in hardware to be able to form a switch (L2) from individual ports. As a router, a FGT supports one subnet per port - there cannot be any duplicate addresses on different ports. Therefore, if you need more than one local port, e.g. in a small office environment, you can use some ports in a switch.
Switch ports and aggregation are different things: traffic is broadcast to all switch ports (one broadcast domain) whereas link aggregation splits traffic in Layer 3 (hash over src or dst address(es)) to achieve higher bandwidth or redundancy.
Even in case your FGT does not have hardware switch circuit you can create a 'software switch'. Be warned, all traffic across a sw switch will be handled by the CPU, and is not offloaded onto the network ASIC. With small models (< 100D/E) this can significantly reduce performance.
"Kernel panic: Aiee, killing interrupt handler!"