I can't speak generally about all of 3blue1brown's content, but the essence of linear algebra made concepts like eigenvectors and values and what they are click for me, there is no question.
I agree with the GP that you really need to work through a lot of problems to fully understand a mathematical subject. But there are a couple cases where I did have real, lasting moments of insight, and the 3B1B linear algebra series was one of them. Hearing it stated directly that the columns of a transformation matrix are the coordinates of the basis vectors after the transformation (a fact that I had somehow missed across multiple linear algebra classes) turned on a light bulb in my head.
The Borsuk-Ulam theorem is another example of something that seems obvious in hindsight but that I had never run across before.
For me it certainly helped, but only as "additional course material". When I was studying for the LinAlg exam I used the script of my professor and tried to understand chapter by chapter.
Afterwards I watched those videos to "check" if I had really understood.
