It can be easy forget what went into reaching the present, one's accomplishments and struggles, and to lose track of where one intended to go and why. Different systems work for different people. Spiral notebook, legal pads, bound book. One copy at home and another at work. Read back through it periodically.
Some people make a monthly summary for easy reference.
Some related ideas a sheaf of small projects sketches - one or two pages of formally presented ideas on a single subject. Bailey A Letter to Research Students ,Duane A. Bailey. A Guide for Students and Advisors .