Why do I pay an extra fee for ipad?

To be clear the Cellular iPad does not mean you can make cell calls or receive SMS text messages over cellular, it is simply a Data Plan that is used on those iPads to allow browsing when not connected to WiFi.

You can buy either a Cellular iPad to allow browsing when not connected to WiFi or a WiFi iPad that can only be used for browsing when connected to WiFi. The choice is yours. Depending on the model you choose, you can save between $150 to $200 when buying a WiFi only iPad and not need to purchase any additional cell plan.

As outlined by Mac Jim ID, having a WiFi+Cellular model iPad does not obligate you to adding a Cellular Data plan to your iPad - although doing so does provide the ability to directly connect to the internet while you are away from an accessible WiFi network.

While purchasing a WiFi Only iPad can save some initial cost (WiFi+Cellular models are slightly more expensive), many Users choose a WiFi+Cellular model iPad for two principal reasons...

1) You retain the ability to use a Cellular connection if it needed - simply adding an active Cellular plan when required.

2) WiFi+Cellular models include GNSS/GPS functionality that is not included with WiFi Only models of iPad. GNSS/GPS provides precise location services which may be important to you if you use mapping - or need an accurate location.

If you have an iPhone or other mobile phone that includes hotspot functionality in your phone's Cellular plan, any model of iPad can access the phone's WiFi hotspot to access the internet while away from WiFi. However, if your iPad needs GNSS/GPS, the WiFi+Cellular model provides capabilities not found on WiFi Only models.

You don't have to buy a new wifi-only iPad. You also have the option to drop the cellular service for your existing iPad from your cell plan (assuming your carrier contract allows it). You would need to use wifi to access the internet, but your GPS would continue to work.