Chimera

I bought this weird "WeMos Mega" combo board from Banggood on a whim. It's an Arduino Mega 2560 clone with an onboard ESP8266. They both operate independently but you can flip some DIP switches to connect the serial pins together allowing them to send messages back and forth. I assume there's some level shifting onboard to match the 3.3V/5V levels too. You don't have to link the two processors, they can in principle leave them disconnected although with a single USB serial you still need to use the DIP switch to set which one you're programming.

As the Mega has four serial ports it gives you the option flick a switch and connect the ESP8266 to Serial3 leaving Serial0 for USB comms/programming the Mega. There's also a separate pin header giving you access to the ESP8266 pins directly.

This is either the best of both worlds, or the worst of both. I've got a project coming up where I would like to use a few more pins than an ESP8266 has as standard and I grabbed it for that. It could easily be argued an ESP32 board with all the pins broken out would cover the same ground.

What's the target market? I'm unsure, I guess people who are used to old-school Arduino kit and want to use the ESP8266 in its somewhat old-fashioned role as a Wi-Fi co-processor rather than as the really useful microcontroller it is in its own right.