Sebastian Gutierrez, in an article titled Math Machine, explains a series of benefits that kids gain from having a notebook dedicated to learning math: kids can refer to previous learnings, inspect what worked and what didn't, and get a sense of progress by seeing what they've gone through.
Here are my favorite recommendations from the article.
Use medium-sized notebooks with a medium thickness (not too few, not too many pages)
Single-spaced, lined pages work best; avoid dotted or gridded notebooks
Numbered pages are great for indexing
Reserve one or two pages at the beginning for an index—reference pages by number and add relevant formulas
Parents should keep their own notebooks to avoid writing in the kids' notebooks
Name each notebook—by naming it, kids can hold power over the notebook itself and the math within, and it's also fun
Customizing notebooks helps kids like them more—the notebook's name, the kids' name, stickers, drawings, and other personalizations
It's great for a parent to be able to go back and see how their kid wrote six months in the past, how they've evolved, etc.
Math Machine is the name one of the author's kids gave to his math notebook.