Why speeding up (C)Python matters
The simple answer to why speeding up Python matters is that speed creates freedom.
Right now there are unquestionably a lot of things that you can't do in (C)Python because they're too slow. Some of these are large macro things like certain sorts of programs; others are more micro things like use complicated data structures or do versions of things implemented in C. Speeding up Python creates the straightforward freedom to do more and more of these things.
In turn, this creates another freedom, the freedom to write code without having to ask yourself 'is this going to be too slow because of Python?'. Again, this is both at the macro level of what problems you tackle in Python and the micro level of how you attack problems. With speed comes the freedom to write code in whatever way is natural, to use whatever data structure is right for the problem, and so on. You don't have to contort your code to optimize for what your Python environment makes fast (in CPython, C level modules; in PyPy, whatever the JIT can do a good job recognizing and translating).
There is a whole universe of Python code that reasonably experienced CPython programmers know you simply don't write because it would be too slow, create too much garbage, and so on. Speed creates the freedom to use more and more of that universe too, in addition to the Python universe that we already program in. This freedom matters even if none of the programs we write are ever CPU-bound to any particular degree.
(Admittedly there's a peculiar kind of freedom that comes from having a slow language, but that's another entry.)
(This is not particularly novel and I've orbited the core of this thought in earlier entries. I just feel like writing it down explicitly today after writing about why I care about how fast CPython is, partly because a rejoinder to that entry is 'why do we care if CPython isn't really fast?'.)