My understanding that as the rolls of thee mills/producers wore out, some sizes got a little thicker and heavier. W6x8.5 became W6x9, w10x21 became W10x22, etc.
The only time I have ever used them was in a dry bulk handling facility supporting different process control equipment off of larger steel towers. Light equipment support type.
I think these small sections are difficult to use because of the connections.
You are forced to weld a lot because the limited web height makes bolted connections difficult.
On my misc. steel work platforms, I rarely go below a W8 or C7 section. That's a small as you can get and still use normal bolted angle shear connections.
