honest take after 2 years of logging: it's a filing system, not an experiment
After a couple of years of keeping one, the value has been almost entirely things I didn't expect.
Useful for: reconstructing what I actually did months ago, noticing I'd forgotten something, not repeating a piece of reading, knowing which vial is which.
Not useful for: inferring anything.
I think that's the honest expectation to set for anyone starting one. Filing system, not experiment.