If you always have your iPhone nearby, then the GPS only model will do EVERYTHING the cellular model will do, except it will use the nearby iPhone's radios. And it cost less up front, plus does not have a monthly carrier charge, and you can use any cellular provider for your iPhone that you want, since the iPhone's cellular provider does not know you have an Apple Watch and is not providing any special services for the Apple Watch.
A cellular model allows you to leave your iPhone at home, and still make and receive phone calls, get internet data, messages, etc... when you leave the house.
Things I do with my GPS Only Series 4 Apple Watch:
• It tells time, date, day of the week
• Every iPhone alarm, appears on my wrist, where you can acknowledge it, dismiss it, etc…
• I like the iPhone 3rd party “Alarmed” app (http://yoctoville.com), for my alarms
• Ring.com door bell can display a static image on the Apple Watch
• Weather (this gets used a lot)
• MyRadar 3rd party app will display radar image
• MyRadar and WeatherBug 3rd party apps will send you weather alerts
• Make/Receive phone calls
• Make/Receive FaceTime audio calls
• Use Walkie-Talkie to other Apple Watches anywhere on the planet
• Silence phone ringing by pressing the digital crown (great for SPAM calls)
• Send & receive iMessages
• Send & receive SMS TEXT
• Say “Hey Siri, 14 and a half minute timer”
• Say “Hey Siri, remind me when I get home to do xyz”
• Say “Hey Siri, message my wife/husband/Susan/George/etc…”
• Say “Hey Siri, start Outdoor Walk”
• Say “Hey Siri, end workout”
• Say "Hey Siri, find my iPhone"
• Say "Hey Siri, flashlight"
• Use a 3rd party garage door opener app and device (such as MyQ)
• You can check your calendar
• Read your email
• If you need to take an iPhone picture where you cannot see the screen, use Apple Watch Camera app as a view finder/shutter release
• 3rd party Ocean's Tide app for beach visits
• Hand washing reminder and timer
• Unlock your Mac (I do this several times a day)
• Unlock your iPhone when you are wearing a mask
• Track your exercise and physical activities
Since my Wife got her Apple Watch, we have been able to reliably iMessage each other, as she never keeps here iPhone on her person.
If my Apple Watch was lost, stolen, or broken, I would order a replacement as fast as possible, as I would miss it so much.