What would be the intended result of crashing comets into Mars? Do comets have enough mass that the addition of that much dry ice, regular ice, and assorted ru

What would be the intended result of crashing comets into Mars? Do comets have enough mass that the addition of that much dry ice, regular ice, and assorted rubble would make a noticeable change in the climate, or is the goal to use the re-entry heat to warm things up? And would a large impact cause a dust cloud that would take years/decades to settle? (Which could be an acceptable side effect if the benefits are worth it, or if the dust itself creates a warming effect.)