In the example there is 8GB installed and usage is normally under 4GB. You said “doesn’t mean you won’t benefit from more”. Unless you use that unused 4GB, installing additional RAM is not very useful.

So, to push the metaphor, if my keyboard and mouse fit fine on the 4GB half of the desk and the other 4GB is enough for the other things I want to do, then 8GB is enough and I won’t benefit from more. When 4GB is not enough, then more RAM makes sense.