Fluff-wise, it's a freebootin', world-hoppin', fungus-beer-drinkin' mob of orks that have left their separate clans to join Azbag da Stomper'er's space-farin' Waaagh!
I like painting all of the different clans and thought, "Why bog myself down with one of the six clans, when I can paint stuff from each one?" Hence, the freebooter theme.
Also, I thought, why just have squads of each clan (a squad of Deathskull lootaz, a squad of Evil Sunz bikerz) when I can mix em all up? So, my solution was to create the following table and roll a D6 for each model in the unit, to decide what clan the model originally came from, then paint it accordingly. It's kept it fun and kept me interested in continuing to find new and interesting ways to convert and paint the models once I knew where they came from.