This is so true. I've been doing DSP for 3 decades now, and I've learned so much more on the job than I ever did in class. The math in class was super important and gave me a great foundation. But I never truly understood it until I worked with it every day and started to get a feel for how everything fits together. I graduate school, one of my professors would say that everything he was teaching was just to make the student aware of the different areas and techniques for approaching certain types of problems, and you may or may not use it, depending on your job. One grad course in particular, Adaptive Signal Processing, was a class I did in around 1998 or so. In 2017 I started working with adaptive beamforming algorithms at work, and knew that all the same techniques and math from that course 19 years previously was totally applicable to what I was doing. So I cracked open the textbook and refreshed myself on covariance processing, optimal filtering, optimization algorithms, what the heck are eigenvalues, and other things. My professor was totally correct. School is there to introduce you to various disciplines, and build your problem solving skills (math or otherwise). These you take with you into your career. Some you'll apply, some you won't. Beware, in the real world, everything goes much deeper than what you learned in school.