Why does the code have provisions made for a certain minimum percentage of reinforcement in the RCC

Well, main reason behind this is to increase the overall durability of the structure.Even though in fully compressive member or member in which the concrete content is sufficient to withhold the load, at certain points during its life, some tensile stresses which might develop due to causes like temperature variations, creep, shrinkage etc.Thus to resist this unforesson / unpredictible tensile stresses some amount of steen reinforcement is provided in structure, this steel in known as Minium steel.