> I mean, hopefully why you might need multiprocessing in python is clear?

My experience, after first getting into python, was:

I needed to do something concurrently on one set of data.

Python threading doesn't provide concurrent execution, so my program slowed down when I used threads.

So, I tried multiprocessing. My program slowed down even more, because any communication between processes uses pickle. I was trying to process one dataset in parallel, and pass big chunks back, for a final processing.

So, I saved it to disk, loaded the dataset into each process, multiplying my memory usage by 16x.

I then threw it all out and wrote the performant bits in C++, using swig to automagically make the python interface for it.

So, knowing why (concurrency) isn't necessarily enough.