That's one use case scenario, but there's also the use case for a standard print. For instance, if you print a 25 mm cube and notice that it's 24.8 mm in the X-Y axis, then you print a 50 mm cube and it's 49.8 mm, you could add .1 mm to the size compensation to quickly remedy the size of your prints.
The horizontal size compensation is a new feature in version 2.2.2, if you don't see this option under the Other tab, this may mean you are using version 2.2.1 or earlier. There are many, many usage case scenarios for which you could use this feature. Taken from the blog post, the gear bearings by Emmet. If you print these out, and the plastic has molded together, you could place a value of -.05 to increase the gap by 1. mm, or -.1 mm gap to increase the gap by .2 mm (half the average nozzle diameter for perspective). the gap is doubled, since both perimeters are compensating away from one another, so the gap is twice the value of the input.