I have two 27" 4k monitors and don't want to waste a single pixel.
I did not looked how the gap feature can be enabled/disabled and I really hope the default is disable contrary to the new "drag window" feature that can not be disabled and that regularly disrupt my workflow.
(I know that an upgrade add the ability to disable the drag window feature, it's just not available in FreeBSD yet).
As someone focused on productivity, gaps help me make sense the current state of the desktop, by separating better the contents of multiple windows in the same workspace.
For single window workspaces you just don't apply the gap (smart gaps).
I use gaps in XMonad and I find they help to make screen borders (or shading in case I use a compositor) more visible to quickly move my eye to the active window.
I find that using gaps (not big ones, 5-10 pixels at most) helps me differentiate between similar windows (like terminals) much easier. And, also, I just looks more elegant imho.