In conjunction with cards you would associate with an Aristocrats deck, Sek'Kuar can provide a great deal of value from already useful cards
You can only use each effect of "Eria the Water Charmer, Gentle" once per turn
Dark Law is so strong that Burning Abyss and other dark decks have played Mask Change II just to summon him
I've written pretty extensively as to why he would be a good commander in that article, my own name aside, but as a refresher, Joshua is relatively novel
Use Yawgmoths first ability to cover your opponents creatures in -1/-1 counters and then proliferate to build those counters up until they reduce threats to nothing
As an example, some near-mint (NM) sales clock in at $9.95, while others jump to $10.94 and $12.30