On the NUCLEO boards you can plugin many cards compatible with Arduino (Uno rev3) but also STM has released several expansion cards (X-NUCLEO) ranging from radio modules to mems to motor controls. The NUCLEO boards are compatible with the Arduino connectors ( Arduino Uno rev3) and there are three models that are 32pin, 64pin and 144pin. To develop on STM32 there are three categories of evaboard that always contain the emulator for debugging and programming the STM32. STM32 is a family of MCU based on Cortex Mx that ranges from Cortex M0 up to Cortex M7.
If I have intrigued you and you are still reading, here is what you have available using the family and development boards based on STM32. In any case, those who use Arduino, will have already noticed that new Arduino boards have been created that mount more performing MCUs (Cortex Mx) just to keep the Arduino platform updated with the current times which require: low consumption, performance and Internet connection. I definitely do not want to convince anyone that it is time to move to a more modern and performing MCU family ( Cortex Mx) but, if you found yourself in situations where you wanted to do more, then the STM32 family with its NUCLEO boards and mBed are the answer you have your wishes. The answer to the above question is simple, for the computing power and the compatibility with the Arduino connectors.