The Phone Plus 3 Months Free offer. Explain.

This limited time offer is available to new customers and gives you 3 free months (a $45 value of 4GB/month) when you buy iPhone 12 mini, iPhone 12, iPhone 13 mini, iPhone 13, iPhone 13 Pro or iPhone 13 Pro Max. It also allows you to upgrade your 4GB plan to a larger plan and only pay the difference.