You want to study the effect of a certain trait on a group. So you take two groups and try to make them as even as possible, aside from the trait you are testing. Ideally you'd match every guy in your sample with someone who is mostly identical in upbringing, location, etc, but taller. That's how you make sure it's actually height causing the effect and not something else. If you don't account for this, you might end up with a bunch of bourgeois short kings getting compared to 6 foot tall poor people, and might even come the conclusion that height has a negative correlation with wealth.
It's not saying "height matters, not those other things." But rather "height is one of those things that effect socioeconomic status, statistically."