In choosing a WiFi+Cellular model iPad, from a technical perspective, you are not committed to having an active Cellular plan - but can add a cellular plan at any time of your choosing. Many users opt for the WiFi+Cellular model as these models include GNSS/GPS positioning functionality that is not available with WiFi Only models.
If you are purchasing your iPad from a Cellular Carrier - and not directly from Apple or a regular Apple retailer - the Carrier (e.g., AT&T) may require you to also purchase and maintain a Cellular Plan. What is likely to be happening is that the iPad purchase may appear to be cheaper with the cost being spread over the life of the associated (carrier-mandated) Cellular plan. For some, this option may be financially attractive, although (a) the overall cost may be much greater, and (b) lacks flexibility.
Be aware that this arrangement also has a hidden cost - in that the iPad itself may be subsidy-locked to the Carrier, such that you’ll be unable to subsequently change to a different Carrier at a later date. In addition, for iPad models that have a physical SIM-slot, the internal eSIM may be permanently disabled by the supplying carrier.
In summary, if considering purchase of an iPad from a Cellular Carrier, be sure to understand the [potentially unseen] limitations and costs. While you may be able to pay for the subsidy lock to be removed at a later date (typically when your minimum contract period ends), it is impossible to reverse a carrier-disabled the eSIM. Devices purchased directly from Apple are “unlocked” and usable with any supported Cellular Carrier.
Given your question, you would be best advised to purchase from Apple - and not from AT&T.