Even the dumbest network card still needs to do "stuff", starting with recognizing its own MAC (of course you can outsource this to a driver in the OS and run it on the main CPU, some kind of "winmodem network card" but it would be extremely wasteful). Of course they do many other things even for low-end motherboards like checksum offload, WOL, etc.
Better cards have some really nifty features like layer 3 firewalls or lights-out management.