What does the nano SIM card do in iPad Pro?

The iPad Pros have an embedded Apple sim you can use to set up Cell service on one of the supported carriers.

And also have a separate sim card slot to use service from other carriers not supported by the embedded Apple Sim.

Use Apple SIM with Wi-Fi + Cellular models of iPad - Apple Support

If you buy a Wifi only model of the iPad Pro it will not have either an embedded sim card nor a sim card slot. And as such will not be able to connect a cellular carrier's network for internet access. Only Wifi networks.

If you buy a Wifi+Cellular model it will have the embedded sim card and the slot, but there is no requirement to have cellular service active with it. It can use wifi networks normally without a sim card, or using the embedded sim with a supported carrier.

Do note that the Wifi + Cellular models are more expensive than the Wifi only models.

iPad - Compare Models - Apple

It lets the iPad get cellular service for internet access when away from a wifi network. Much like a cell phone does.

Note that it only enables data access. No voice and text, so no sms texting or cell calls can be made directly from an iPad.

Artemisalm wrote:

The new iPad pros seem to have a built-in SIM card plus the nano SIM card. Why are there two of them. I don't have an iPad Pro yet but am considering it. I will probably get wifi only. Is the SIM card necessary to use it with wifi?

So you can use SIM card from a mobile (or cellular) data provider that does not have partnership with Apple. No, you don't need a SIM card to use WiFi. WiFi only device doesn't have SIM slot or built-in SIM.

It allows you connect the mobile phone network