For reference, the msg parameter of Task.cancel() was added in issue31033.

It seems that the initial use case was for debugging. I do not see how it differs from the following example:

    r = random.random()
    if r < 0.5:
        x = 0
    else:
        x = 0
    1/x

In the traceback we see the line where an error occurred but we do not see a line which lead to this error.