Take sleep tracking: I tried and failed to note those times when I wake up. I'm usually too foggy to remember, and later in the day, it's too much effort to reverse the times out of environmental cues. Obviously, I don't have a consistent sleep schedule - otherwise, I wouldn't care about tracking it in the first place. And, of course, manual tracking doesn't work when I wake up in the middle of the night and go back to sleep an hour later, because half the time I don't remember it happened until next afternoon. However, my smartwatch is able to record all that for me, with 95+% accuracy, which over the past year or two gave me some important data to contextualize my day-to-day energy levels.
Or take pressure monitoring. My whole life, I've been suffering from acute chronic cannot-form-a-habit-to-save-my-life-itis, (now also known as "treatment-resistant part of my ADHD"). There's no way in hell I can stick to daily (or worse, less frequent) measurements using cuff-style upper arm monitor. It may be portable, battery-powered, trivial to use, but it's still too much of a hassle. What I can do is use it to recalibrate my smartwatch every couple weeks, and use that to take measurements - I still can't form a habit, but at least something I can do within couple seconds of remembering it. So, thanks to the watch, I get some data, where otherwise I'd have none, and that's all because of convenience factor.
Or just look at the popularity of fitness bands and watches. Some people derive value from those automatic measurements. Of those people, many (most) wouldn't otherwise bother if they had to use a stopwatch and a notebook instead.