I can come up with several reasons for concealment of a PSK.

1. Social. I know that I break out in hives when I don't have a leatherman tool at least near me, but suit pants just don't really cut it for carrying gear.

2. Security. PSK's have "interesting" stuff in them. If I was in a place where interesting stuff might attract too much attention, I'd not want something as obvious as a waist pack on my person.

3. Urban Environment. The IED and Car Bomb are two very real things, and both can leave you entrapped in a building for a while. You might not need it at hand, but to have gear with you isn't a bad idea.

4. It's cool.