I am not entirely sure what it does, but it feels like turning on "linear scale" spreads volume (over 100 levels) much more evenly if that makes sense.

For example:

When I use apple dongle for my iems, I usually gets comfortable volume around 3-5 steps of volume level. But increase in one more step of volume level from this makes it painfully loud without linear scale turned on.

With linear scale turned on I usually end up in comfortable volume around 60-70 steps. And increase in each steps of volume level is not making it painfully loud.

So I get much more flexibility in adjusting volume to taste with linear scale on.