A large part of my audience over at Math ∩ Programming are industry software engineers who are discovering two things about mathematics: it’s really hard and it opens the door to a world of new ideas. In that way it’s a lot like learning to read. Once you’re mildly fluent you can read books, use the ideas to solve problems, and maybe even write an original piece of your own.
Many people who are in this position, trying to learn mathematics on their own, have roughly two approaches. The first is to learn only the things that you need for the applications you’re interested in. There’s nothing wrong with it, but it’s akin to learning just enough vocabulary to fill out your tax forms. It’s often too specialized to give you a good understanding of how to apply the key ideas elsewhere. A common example is learning very specific linear algebra facts in order to understand a particular machine learning algorithm. It’s a commendable effort and undoubtedly useful, but in my experience this route makes you start from scratch in every new application.