To my knowledge, these boards will work independently of a microcontroller that would produce TCP/IP packets. So in a sense, you to have TCP/IP to serial bridges.
You are correct though, they are protocols and you really need to layer protocols on top of TCP for it to be of any value (eg. telnet or HTTP). At this point, I don't know how these bridges are wrapping data so I don't know if they will be useful.
As for equant not knowing why PPP ain't sufficient, well, I can imagine one case: if you want to be able to connect to the internet independent of other computers (in your home). Which is kinda hard if your using DSL or cable services.