Do you mean, why do ta's pre-cook chicken in sauce instead of just cooking chicken by frying, roasting or boiling?
If that is the question then the answer will almost certainly vary by region.
Some places will do it for taste, texture, colour or even to save on the gas bill as its simple to cook the meat in the base sauce. It is also much less likely to over or undercook the meat, especially with lamb that varies so much in tenderness.
My local experimented with slicing its chicken thinly and cooking it fresh to order a few years back but the result was rubbery chicken slices so they went back to their old ways.