Without a cellular data plan, a smartphone isn't very smart, unless you're near a Wi-Fi hotspot. In fact, when you buy most smartphones, a cellular data plan is mandatory. And, as you know, data plans don't come cheap, even if you try my tricks for saving on your cellular bill.
That's why it feels like a waste when you don't use all your monthly data and everything resets when the next month rolls around. You're basically paying for something you didn't use.
You could always switch to a lower data plan - learn the mistakes to avoid when picking a new one - but maybe some months you do use all the data. Do you have to resign yourself to wasting data some months?
Not if T-Mobile has anything to say about it. It just announced a new feature that does something carriers should have been doing all along.