The need depends on your usage. The Watch is capable of a lot but, need, has limitations.
I primarily use mine to check time, secondly notifications, and third fitness. I have used replied via Watch a few times, but the task is tedious. Speaking of tedious, I would not want to routinely read full news articles, surf the Web, watch videos, etc on such a small screen.
I think the assumption is if someone checks the time (more than once), they are thinking, “how much longer?” or “is it over yet?"
The cellular version is helpful if you don’t want or forget your iPhone but yet need/want to be reachable by others, emergency contact, weather check, etc. However, as convenient/helpful as that functionality is, in my opinion, it’s nice to get away from frequent notifications/alerts for a period.
By the way, I don’t have a cellular Watch because of the added cost and my wireless carrier doesn’t currently support/offer Apple Watch plans, as far as I am aware.