Remember this guy?
Before we get to meat of this post, let's join Peabody and Sherman and ride the WABAC machine back to Vanilla wow.
Back then, during the less than Ret favorable days, it would auto crit for double damage when the target was stunned. If a ret paladin was fortunate enough to get the Field Marshall's Aegis, he could do some serious damage as either Ret or Protection... provided it was the famous tri-spec build you used previous to 1.9.
It also had a serious drawback. Considering the wording of the original seal state that it had "a chance" to deal additional damage, it was given the moniker "Seal of Casino". If one was fortunate, one could hit targets for massive damage. If not, you'll get a white swing and nothing more until it was judged and the seal reapplied for another attempt.
Fast forward to Lich King Times, where, after the removal of Seal of the Martyr and changing Seal of Vengeance to work on auto-attack swings only, the developers hoped that by making Seal of Command actually hit with every swing, we would choose that and still have a chance at pvp.
Nice idea, but the damage had been utterly gutted and hit like a wet noodle by this point in time, by several nerfs between Vanilla and now, and any ret paladin worth their salt would have to use Seal of Righteousness if they wanted to make any headway, considering they still had no snare, interrupt, or means to deal with casters (but that's for another time).
In a stroke of genius, or madness (take your pick), the developers decided to turn the seal into a cleave.
The damage by itself was nothing special, but the fact that it automatically hit three targets at once, when used in conjunction for both Hammer of the Righteous and Divine Storm*, resulted in much more AoE damage for Rets and Threat for Prots.
*Though Divine Storm will proc the seal on the attack itself, it won't proc the cleave, so, you're not hitting as many targets as we would have hoped.
As an example; for ret, let's take my current weapon Shadow's Edge without the two Bold Dragon's Eyes and enchant it with Berserking I have on it.
Here's the math:
Without all that stuff added on, it will do somewhere within the range of 313.92 - 471.24 damage per second. And it will hit four targets when used with Divine Storm. Add in the 2 piece t10 proc and you got lots of AoE damage going around... when it procs, of course.
Now, take all that and multiply it by three (four if you use the glyph) for Hammer of the Righteous, add in the amount of threat you get from Righteous Fury and you got pretty much the best AoE tanking seal in the game at present.
As a side note, it would have been nice to acquire Seals of the Pure for extra damage and threat when tanking, but it doesn't affect command, so, it's left to its lonesome.
So, there you go. Seal of Command. From Slacker to Booster, thanks ! (Fast forward to 10:08).