The GPS only Apple Watch can do Everything the cellular model can do as long as your iPhone is nearby, where nearby is somewhere in the same 3-4 bedroom home (assuming typical wood frame construction). And of course when the iPhone is with you when you leave the house.
https://support.apple.com/en-us/HT205547
The Cellular Apple Watch allows you to leave your iPhone at home while you go for a walk, run, bike ride, etc...
The Cellular Apple Watch shares the iPhone's phone number. This requires the iPhone cellular carrier to support the Apple Watch. The iPhone carrier charges a monthly fee for this service. In the U.S. the fee is $10 per month added to your iPhone bill.
iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:
https://www.apple.com/watch/cellular
PS. I have GPS only Apple Watch, and as I always have my iPhone nearby, it does everything I need. My Wife has the same Apple Watch, and often her iPhone is in the kitchen, and she frequently answers phone calls on her Apple Watch, and sends iMessages to me and other family members from her Apple Watch.