In the past I have used certain makeup/skincare products and have taken a long time to figure out that a common ingredient among all of them was breaking me out. It also helped me figure out that certain foods containing this ingredient broke me out as well.

It would be cool to look at the calendar and be able to see “Hey, most of the days after I used product A and days after I used B I developed painful acne, and both those products have talc, maybe that irritates my skin” or “My skin is always great on days I don’t eat dairy but worsens when I do, maybe I should cut back”. Other skin concerns like flaking skin, oily skin, eczema, rosacea, etc would be great as well.

A makeup/skincare section could be similar to the medication section, where you add the name of the makeup/skincare (with a notes section where one manually could input the ingredients inside it), and when/how often during the day it was applied.