The Burmese lack the Siege Onager and the Siege Ram. Manipur Cavalry powers up the Burmese Battle Elephants' raiding capabilities so they can compensate for that.
It must be mentioned that, as of patch 5.2, the +6 damage is split as +3 against the "standard building" armor class, and +3 against the "building" armor class. Masonry gives all buildings +3 armor in the "building" armor class, nullifying half of the effect of Manipur Cavalry. The exception to this is the Battle Elephant unit, because it already has bonus damage against the "building" armor class, and therefore enjoys the full +6 damage against buildings.
A team containing Berbers: With Kasbah researched, researching Manipur Cavalry is 25% faster.