They would be very useful for small trees, and to add texture to custom trees.

I would even push it further:
A recipe OUT of Bark blocks like stairs, slabs and walls to get planks out of them.

The reason could be to make the blocks more survival friendly. I never liked the idea that tree would be made of blocks which isn't even wood... and then you put that creation in world gen!

If you were able to craft planks out of all Bark blocks, at least it would slightly justify using those bark blocks in an addon which would perhaps spawn modified trees.