Lorewise they're the equivalent of the 40k adage with the hammer of the emperor and the spear. A paladin is the hammer of his order: big, clumsy and imprecise, despite being effective at what he does. The avenger is the spear: precise, controlled. He finds the targets threatening his order/religion/sensibilities and takes it down.
Mechanically they're fast as fuck, hit hard and are near impossible to run away from. They're assassins in the sense that Altair from the AC series was an 'assassin'. You run up to the man you want dead and start hitting him. If he runs off, you chase him down and keep hitting him. He's unlikely to win a duel because you're a damn good duellist and have god on your side.
In short, they're some anime bullshit but I love them.