iPad sim card

If you intend to use cellular services for data on your iPad, you would need a sim card. If you don't want or need to pay for data services on your iPad, you don't need one.

iPad's which are Wifi + Cellular permit you buy a data plan for your iPad, so you can still get online when you are in places without access to Wifi. It does not turn an iPad into a cell phone as iPad does not have a cellular radio like a cell phone.

With the SIM installed, you get data allowance where you can connect to the internet without Wi-Fi.

You cannot make calls unless you also have an iPhone.