if you do it on graph paper with each
menstrual month on one line, it comes out like a bar
chart after a while to show you the normal lengths of your bleeding, when you ovulate, and when you have vaginal secretions of other kinds. That can be useful for projecting forward to see when your next period will probably come, and things like that.
Going on the pill can change the quantity of discharge, but it does not always do so.