What can’t I do with iPad+cellular if I don’t insert a SIM card?

The ONLY thing that will accurately work with a WiFi AND cellular data iPad is the Apple Maps and Google Maps functionality.

Cellular data iPads have built-in GPS antennas.

So, without a SIM card, your cellular data iPad can act as a highly accurate GPS device.

Great for traveling IF you need both a GPS AND your iPad.

You can use WiFi anywhere but you will not be able to use cellular data without a SIM card and cellular plan of some sort.

You will be unable to use cellular data.

You can still use the iPad on WiFi anywhere. However, without a SIM card and cellular plan, you will be unable to use cellular data.

No. Again, you need a SIM card and a data plan to access cellular service anywhere. If you have that, then you can use cellular data. If you don't, you can still use WiFi.