Regular trains, especially the popular ones happen to be dailies with rake consisting of 20+ coaches in many cases. such trains require dozens of coaches for LHB-fication, and trains like these exist in every zone which makes it difficult to meet the demand across the country. So when the zone starts getting LHB coaches they usually go for weekly train to make LHB, but rest of the coaches received can't be just kept idling as there is significant amount of pressure from RB to improve coach utilization, which is where these Special trains enter into picture.