A one-way slab is analysed for bending in one direction, so a reasonable first instinct is to put steel in that direction and distribution steel across it as an afterthought. IS 456 does not treat it as an afterthought.
Clause 26.5.2.1 requires reinforcement in either direction of not less than 0.12 per cent of the total cross-sectional area when using high-strength deformed bars of grade Fe 415 or Fe 500, and 0.15 per cent when using mild steel. The percentage is on the gross section, not the effective section, and it applies whatever the bending analysis returned.
The reason is that the analysis models load. It does not model the concrete shrinking as it cures, the slab expanding and contracting through a forty-degree annual swing, or the restraint offered by the beams and walls it is cast against. Those actions produce tension in directions no load case covers, and cracking follows the weakest path available.
Minimum steel does not stop that cracking. It distributes it — turning one wide crack into a set of fine ones, which is the difference between a durability problem and a cosmetic one, because crack width governs how quickly chlorides and carbon dioxide reach the bar.
Two practical consequences. Distribution steel is a code minimum, not spare capacity to trade away when the bar schedule looks expensive. And the 0.12 per cent figure assumes deformed bars: a detail that quietly substitutes plain mild steel needs 0.15 per cent, a twenty-five per cent increase that nobody remembers to apply.