You can discover your weaknesses yourself by doing problem sets then checking solutions. You'll notice what kinds of questions you keep getting wrong, then you make a note to study that area again or you do more problems in that area. You don't need a computer algorithm for this.

Right, and then you’re expending mental energy on figuring out how to teach math (to yourself) instead of on the math itself. This is not wrong, and will likely even teach you a thing or two (and in fact it was how self-teaching math worked before this came along) but, to me at least, MathAcademy seems to be more efficient in getting you to do just the math and nothing else.

You don't NEED anything really. But it's helpful to have a computer algorithm for this. Processing that yourself is meta effort not everyone has the extra time for or will be diligent enough about.