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.