IIRC, specifically they have a slightly ranged camel-type attack(strong vs horse-cav), which is also considered a physical-type attack, making them quite powerful vs siege weapons, too(you can easily kill battering rams through a wall, much as you could with Mangudai).
I remember I loved Elite Mangudai, as in addition to the vs siege bonus, they have the rare trait of 'fire immediately on clicking a target', whereas just about all other archers have a draw/reload animation that has to finish first(the only other that springs to mind is the Elite Chu Ko Nu(the Chinese repeating crossbow archers)).
