There are 2 models of Apple Watch. The GPS-Only model, and the GPS+Cellular model.
The GPS only, can do the same things the GPS+Cellular model can as long as the paired iPhone is nearby, or both devices are connected the internet. The GPS-Only model can get calls and messages directly from the paired iPhone if its nearby orr connected to Wi-Fi without having to add it to your Sprint plan.
The GPS+Cellular Apple watch can independently connect to cellular service to get calls, and messages, and access the internet while away from the paired iPhone and a Wi-Fi network.
The GPS+Cellular model is the one you can add your Sprint service for cell service. This has an added cost to your Phone plan.