When it comes to curriculum, what matters more than how often you will use some bit of knowledge in your career, is the relative difficulty of learning something particular in a classroom setting v. on your own once you are out of school.
Partial differential equations are definitely easier to learn in a college classroom with the aid of a professor who your tuition dollars are paying for, along with textbooks, TAs, study groups, and time in your schedule to spend considerable time to focus on learning it for many weeks and do problem sets, than it is in the "real world" after you've graduated and either work full-time, or have young children of your own to raise, or both.
Many other things that you need to learn in life, or are just interested in knowing, are much more easily learned on the job, or through self-study in your spare time. It is much easier, for example, to learn how to invest money, or deal with a copying machine or printer that isn't working, or how to file an expense report, through self-study with assistance from co-workers and peers, as an adult who has finished college, than it is to learn PDEs.
A related factor is that some knowledge has a longer shelf life than other things.
For example, financial accounting and tax law, just like PDEs, is much easier to learn in a classroom setting than it is to learn on your own. But unlike learning to solve PDEs, a skill which is forever because the rules for doing so will never change, tax law changes literally every year, and after a couple of decades, can be barely similar to what you learned in school. So, you have no choice but to constantly relearn some subjects as an adult because they are always changing.
Similarly, as an undergraduate in college, I had no idea that I'd spend 20% of my professional life doing advanced typesetting due to the rise of word processing and the demise of administrative staffing. Many professionals were still dictating things they wrote to secretaries when I was an undergraduate. But this has become the reality since I've graduated, and I've just had to learn.