Why chair is provided in footing, slab and raft foundation. What is chair ? Practically knowledge

Details knowledge about chair.
lapping lenght, development lenght. 2 clear cover , nominal cover, effective depth and effective cover. 3 unit weights of steel and derivations 4 how to calculate cement sand and aggregate of M20

function of chair bar in slab/footing. cutting length of chair bar. how to calculate height of chair,