For 99% of multivariable calc students, once the course is over and the social environment moved on, there is no reason or motivation to leaen it. Like most of college coursework, it's just a ritual.
But if you're on HN, you might have an interest in machine learning, in which case it can be extremely helpful.
If you want to work through this material yourself, you may enjoy Hubbard & Hubbard’s book, https://matrixeditions.com/5thUnifiedApproach.html