Smaller devices, and especially phones with dual sims.

Things they could have done with micro sims no problem.
My old phones had dual sim, used a micro sim cards, and were a hell of a lot smaller than my current, nano sim phone.

Agreed! I could see if the technology were radically different, but it is not. It is the same basic electronics. In fact, theoretically, you can cut a micro card and make it a nano..
A waste.

@LuckyGuy
That is what I did to put my old sim card into my new phone.

In fact, I bought a specialised sim card cutter just for that.

you can cut a micro card and make it a nano

This suggests they were probably conceived together, to accommodate both immediate needs, and future tiny-device needs.

Or maybe they were conceived together, to provide an easily handled card and a less convenient teeny card which fits teeny devices.