I think there is some confusion with networking basics going on here.
The reason why your PC needs a static IP in conjunction with port forwarding is based on how your PC is connected to the Internet. Port forwarding is needed when you use a router in between the modem and the PC. More specifically, the router being used is probably a router which does NAT (network address translation.) The reason why NAT is used in all SOHO routers is because it provides the ability to have more than one PC (or network attached device) on your local network to share the same Internet connection as most home Internet services only provide one public IP address. Without NAT, you would only be able to connect one device at a time to the Internet. Port forwarding is a method by which you tell the router based on the incoming port (service) to forward that specific traffic to a specific device on your internal network at a specific IP address. This is why setting a static IP for the target device on your internal network is important to make the port forwarding work consistently and properly.
When you connect a PC directly to the modem, you won't be needing to do any port forwarding as there is no router to worry about. The PC will get a public IP address from the ISP directly and you will be able to directly access the PC. Keep in mind, directly attaching the PC exposes that PC to all sorts of risks of being compromised.
The reason why your PC needs a static IP in conjunction with port forwarding is based on how your PC is connected to the Internet. Port forwarding is needed when you use a router in between the modem and the PC. More specifically, the router being used is probably a router which does NAT (network address translation.) The reason why NAT is used in all SOHO routers is because it provides the ability to have more than one PC (or network attached device) on your local network to share the same Internet connection as most home Internet services only provide one public IP address. Without NAT, you would only be able to connect one device at a time to the Internet. Port forwarding is a method by which you tell the router based on the incoming port (service) to forward that specific traffic to a specific device on your internal network at a specific IP address. This is why setting a static IP for the target device on your internal network is important to make the port forwarding work consistently and properly.
When you connect a PC directly to the modem, you won't be needing to do any port forwarding as there is no router to worry about. The PC will get a public IP address from the ISP directly and you will be able to directly access the PC. Keep in mind, directly attaching the PC exposes that PC to all sorts of risks of being compromised.