If you wish to track sleep, then battery life that lasts several days is important because you don't wish to miss a sleep data point every 1 or 2 days just because you need to charge the Apple Watch overnight and can't wear it to sleep.
It is precisely because of Apple Watch's limited battery life (~1 day for series 0 and about ~2 days for series 4), that I bought a Fitbit Charge 3 to track my sleep and resting heart rate. Fitbit Charge 3 battery lasts 7 days so I have 7 full days of tracking data before a full charge of an hour lasts another 7 days. My fitbit is with me 24x7 and only comes off once a week for a 1 hour charge.
So I wear both an Apple Watch and a Fitbit Charge 3. The Apple Watch for notifications, calendar and other daytime productivity tasks, while the fitbit as my health tracker - tracking my resting heart rate, sleep data, and steps.
I wish for the day when the Apple Watch can last a full week on a single charge but that's a long time from now. Until then, Fitbit fills the gap.
