Why do Cellular iPads get assigned Phone Numbers?

It's a Mobile Device Number, not a phone number. It's how your iPads are able to get cellular service. Without that, it would be wifi only. read more

If you have a 3G/4G iPad, then it will have a phone number associated so that it can connect to your carrier's network. That however doesn't mean that you can make or receive calls or send texts. You can do those things to other iOS devices through FaceTime or iMessage, but not to regular mobiles or landlines. read more

If you own an iPad or iPad 2 WiFi + 3G model and you need to talk to your carrier about anything involving cellular data, you’ll need to know your Cellular Data Number. It’s one of the first things you’ll be asked for by an AT&T or Verizon rep if you contact them for support or questions relating to your 3G data plan. read more