iPad sim card

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.

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.

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.