This is a huge mistake for me! Forgetting your iPhone when you're out and about (restaurants, bars, etc.) and being alerted on your Apple Watch seems like what this "left behind" feature was made for, yet they have left out that ability. Just plain dumb. Hopefully they will add that obvious capability on later.Of course, these alerts depend on you not leaving your iPhone behind (there's currently no equivalent separation alert available on Apple Watch for this scenario, sadly), so that's the device you won't want to forget, wherever it is you go.
The good news is it sort of saves me from buying an Air Tag for my backpack as my iPad Pro (that lives in my backpack) can act as an Air Tag and alert me before I drive away without my backpack!