The GPS only Apple Watch will do EVERYTHING, the cellular model will do AS LONG AS your iPhone is nearby.
The GPS only Apple Watch either uses your home WiFi for internet access, or when away from home, it will talk to your iPhone, which you would need to have with you, and use the iPhone's radios to access the Internet.
You can leave your iPhone at home while you go for a walk, run, bike ride, etc... so you can track your exercise, and when you get back home, the Apple Watch will sync the exercise data with the iPhone Health and Activities apps.
I have had a GPS only Apple Watch for over 5 years, and since I always have my iPhone with me, I can use it for phone calls (I've answered the phone on my Apple Watch while riding my bike multiple times). I can send messages, which I tend to do by saying "Hey Siri, message <insert wife's name here> that I saw a 3 turtles on the pond during my ride", I can set a reminder by saying "Hey Siri, remind me when I get home to take out the trash", and it will set a geolocation reminder that will go off when I get back to my house.
ANYTHING you can do with the cellular Apple Watch, you can do with the GPS Apple Watch as long as your iPhone is nearby.
And the GPS only models are cheaper, and do not have the $10 U.S. per month added to your iPhone cellular bill.