The PIO allows you to bit-bang DVI/HDMI; you can put the RP2040 on any bus imaginable (which is the real reason it's popular for console modding) (from a digital perspective of course); you have two PIOs so you can convert between two buses/signals, etc. You can convert digital video, e.g. RGB as used on consoles or DVI, to VGA or DVI if you wanted to. In the past you would've needed an FPGA or CPLD in the middle.