During the Patriotic War, the Russian army suffered a lack of light cavalry. The only remaining regiments of Hussars, Uhlans and Cossacks weren't enough, especially since a number of dragoon regiments didn't prove to be very useful.
A special type of cavalry was required, more flexible than Hussars and Uhlans. Thus, in 1812, mounted-chasseur forces were created based on the example of Napoleon's mounted chasseurs.
Mounted chasseurs were mostly used to perform mounted reconnaissance, assault supply wagon trains and execute sudden ambushes.
Instead of sabers, they were armed with muskets. Well-trained riders could fire from horseback. That allowed them to effectively cooperate with Uhlans and Cuirassiers, and act as the "last shooters".