It is important somewhere to keep track of your research logic as you progress. Otherwise you might not remember “why” you are researching a certain person.
Several years ago, I focused on a certain Benjamin Butler in the 1850 census as being “mine.” Using that enumeration as the starting point, I searched other records and made research progress–on the apparent family listed in that enumeration. A stack of papers and records. One problem–I didn’t track WHY I thought this 1850 census entry was for the my Benjamin. It took me hours to reconstruct my reason–time wasted.
When I decided the 1850 guy was “mine,” I should have written down my reasons. That would have saved time later and made it easier to review my reasons should that have become necessary.
Writing up your research, even if for yourself and only yourself, is never a waste of time.