Hi, i dont keep any notes & i have 3 aquariums.
As i rule when i do water tests if the stats are not where they need to be, I do what i can to resolve the issue there & then,i.e water change, filter clean etc etc.
I can see though why people do keep notes so guess its just preference. You can keep notes for Tank maintenance (Water Change) so you dont forget what day its due or keep notes for Medications incase your dosing with something or maybe if you have any fry & keep a record of born date(s)
So really to answer your question, Personally i dont see the point of recording stats but for other things yes & its a preference thing