It is not a feature.
Any vibration you are feeling while charging is a byproduct of the process. It can vary from nothing to noticeable depending on minor differences in the components and assembly of the device.
If you pay attention you'll notice it in may electronics and electrical devices; especially power supplies. My iPad's power supply has a mild vibration. My iPad does not.
I am surprised that it is happening in any iOS Device itself. I wonder what circuitry is causing it. The usual cause is the transformer that converts household AC to whatever else is needed, and that is external the iOS devices. Or is it possible that those who are feeling it are charging in a dock or some other method that could be physically transmitting the vibration to the device? Even having both in firm contact with an intermediate like a table might to it.