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 Masonry gives buildings +3 building armor, so half of the bonus is not unlikely to be nullified in later stages of the game.
A team containing Berbers: With Kasbah researched, researching Manipur Cavalry is 25% faster.
Rise of the Rajas
Initially, Manipur Cavalry gives +6 attack against buildings. With patch 5.3, the bonus is spread over the two building classes (Building and Standard Building) equally so Architecture does not cancel the effect completely anymore.