If the phone is ALWAYS with you, then the watch is not a necessity. More a convenience: you can look up time or any other important info and receive calls and urgent messages without pulling out your phone.Since my iPhone is always with me, and AW health features fall short for me, I see no reason to even HAVE an Apple Watch.
While my phone is almost always with me, there are exceptions. For example, I don’t usually swim with the phone, but I can swim with the Apple Watch. It’s not a common occurrence, but I did answer some calls in a swimming pool and was glad I was able to.
There are situations where pulling out a phone is not convenient or is not polite: at business meetings, at a cinema, etc. But glancing at a watch is acceptable.
Again, not a necessity, but certainly a convenience.