Archery based skirmish is the one I think of as most understandable, because I enjoy studying the Mongols and Genghis Khan. The horde's fake retreat tactics are modelled fairly well by skirmish rules. Well, most of the use was to exhaust infantry and heavy cavalry from the chase, then abrubtly turn about and fire a hail of arrows, but still.... Also, some Mongol cavalry archers could fire at enemies as their mounts retreated. Plus, historically "skirmishers" were usually ranged, whether slingers or germanic javalin throwers. As to why D&D represents this combat style as a bonus to damage, who knows? Probably just to be in sync with SA. I suppose if I were pursuing someone, and then they turned and shot me, my defenses would be lower, but all of this conjecture relies on actually being pursued, while in the game the monster can just be standing there.
For melee, I see it like the "cause overreach" feature of the elusive target feat. You move away from the enemy, they move in to take a swipe at you, and then you swiftly dart back in, under the incoming attack, and slam home a powerful direct hit. Of course, the errata says you can't end in the same square as you began, so that tactic isn't allowed. It also disallowed using skirmish from horseback, making my other example pointless. I guess the real lesson is: the errata sucks!