Wow, that jogged an old memory and story. When I took calculus in my senior year of high school (back in those days it was a big deal, not like today where many take AP Calc AB or even BC as juniors!), I didn’t do so well. Yes, there was a girl involved, what can I say. But it did mean that I took Calc 101 as a freshman at Tulane, i.e. started over when it came to calculus. Well, I came to class with my notepad and pencil all ready to go. But then the guy started the lecture, and I just sat back and listened. My test results the previous year notwithstanding, I had learned a lot more than I realized. Upon hearing it again, I was able to really focus on what was being explained, not just memorize formulas and problems. I not only completely aced the course, I only had to work about half the assignments since it just made so much sense now. That pretty much carried over to Calc 2 next semester, although I worked more of the problem sets, about 75% of them. Got in a bit of a tiff with my prof over that (I didn’t realize he was counting homework towards the final grade) but I prevailed since I got nearly 100% on every exam.
The physical reality of what the math described really took hold. That not only made all the other math courses I had to take much richer and easier, but it allowed me to do much better in physics as well. The capper on the story is that my physics prof sophomore year threw in a question every weekly quiz designed to be at the very edge of what had been covered, to see if people really understood the concepts instead of strict memorization. Often that meant being able to set up the solution because you understood how the whole thing came together in the real world, and solve it for the correct numerical result using calculus that everyone was supposed to know because by then everyone was supposed to be through at least Calc 2 and into Calc 3. But most stumbled on that part and only got partial credit for setting it up right, if they did that. I picked up at least 100 extra points during the semester because I was able to apply the right math to the problem at hand and get the correct value at the end. Took me from a B to an A-. It was a tough course.
Was it ever something I personally had to apply outside of school, per the OP’s question? Admittedly rarely in a very direct fashion, since my career didn’t involve those sorts of physics or engineering issues. But as I stated earlier, I sure do appreciate the work of others at a much deeper level than I would otherwise, and that really is quite satisfying. Anyway, thanks for the shove down memory lane. My kids now hate you just a little bit. 
Now that is what I call giving 110% effort.