main question to ask yourself, is there ever a time you want to be connected, but not have your phone.
Do you often find yourself forgetting your phone, either at home when you leave, or in your car when you stop for an errand.
most people tend to have their phone with them all the time, since we're so used to having it in a pocket or purse and not really noticing it.
for things like running, you might not want your phone, so it's not bouncing around in your pocket, so cell on the watch could be nice.
For biking, it's easy enough to stick the phone in a saddle bag, meaning it's still close enough for the watch to connect.
for things like swimming, football, basball or basketball, where you're moving mostly in a smaller area, you could leave your phone on the sidelines in a bag, you might not have connection 100% of the time, depending on which section of the field you're in, but notifications should come in once you get closer to your phone.
If you're thinking about going somewhere without your phone but that place has wifi, then cellular wouldn't be needed. This could apply at the gym if you wanted to leave your phone in your car or locker.
a lot of functions on the watch will work without a connection, with Apple Music and Spotify, you're able to sync music and podcasts to the watch, so you can listen directly from there. you also don't need a connection to record workouts or use Apple Pay.