I recently had a very interesting task to do in which multiprocessing saved me. I basically had to do a lot of requests to a few separate servers, dealing with very small amounts of data, but many requests.
Working with PHP, I did things the old fashioned way, and the best time I obtained after a few hours of work resulted in ~120 seconds to run a certain test (many requests + network delay + no async)
But that wasn't nearly enough compared to what I needed, and after failing miserably with PHPs multiprocessing, I switched to Python.
After a few hours, I had a running Python multiprocessing script that ran in 20 seconds, and after a bit of fiddling with the timeouts and no. of threads to be used, I got it down to ~10 seconds.
This was for a website written 100% in PHP, except a single, 100 line Python script. And the whole thing is working perfect.
My conclusion would be that even if it won't help you on a day to day basis, you may encounter situations where knowing at least the basics of concurrent programming will help you greatly.
Good luck, and happy coding!
PS: I'm not trying to bash PHP, but PHP simply wasn't the right tool for the job at hand.
PS2: Knowing a new technology, or a new way of doing things can open the door to a whole new world of possibilities.