Take a look at this:



As you can see SPI2_MOSI can be accessed at either PC_3 (bottom left) or PC_1 (bottom right), is there any important difference between these choices? I suppose that if someone was using ADC13 that would mean PC_3 isn't available for SPI but PC_1 would be, is that all there is to this?

 

I will start by saying that this is all surmise as I don't know the Nucleo but based on what I do know in general I would say your explanation is correct. By making SPI2_MOSI available in two ways it makes things more flexible. It seems unlikely (though not impossible) there is any qualitative difference.

I only provided this nearly useless answer because no one else had answered and you seem to have worked it out yourself.