And to add to what Wayne said,
As a Modeler, I will leave that Toggle off 99% of the time. It doesn’t mean that you can’t see the effect of the Modifier:. For instance:

And even when re-Topologizing, I tend to leave it off, because I find it mostly distracting. Switching to Object Mode shows both sides and Toggling between the two is something that works a lot better for me, than having that Option enabled all the time.