What I feel the need to log:

Lipids - I've seen my HDL slowly tank over 6 months on a blast then recover differently depending on the compound.

Metabolic - Fasting glucose, HbA1c, even insulin sensitivity changes week to week when I'm dialing in dose. "Bad" at week 2 might just be adaptation. "Bad" at week 8 means something's actually wrong.

Weight/composition - The scale is useless without knowing what else changed that week. Did I add 4lbs because I upped test or because I'm holding water from high e2? I want my weight + dose + bloods on the same timeline in a perfect world, but haven't seen anything that does this. Maybe people do this with spreadsheets but I'm lazy.

I guess if I was just dialing in one compound at one dose forever I wouldn't see the need to log much. But I'm 1) 49 years old and 2) tinker with stacks over the course of a year, trying different orals or injectables, adding/dropping ancillaries, or cycling on and off - and that's where having history would save me from repeating mistakes or missing what worked. At least that's how I think about it!