Yes, it really is free. No, they won't spam you.
Serial ports are a pain.
You can get multi-port boards that address the problem, but they're inelegant. It's a mess. Which is all very well. There's an awful lot of specialised hardware out there that uses a serial interface and may never exist in a USB version. It lets you add another serial port to any USB-compatible Windows computer in about a minute, without taking up any extra system resources or requiring you to open the computer up, or even reboot.