Cover is the layer of concrete between the outermost reinforcement and the surface of the member, and it enters design calculations from two directions at once. On the capacity side it eats into the lever arm: the effective depth d is the overall depth minus the cover, the link diameter, and half the main bar — a 500 mm deep beam with 30 mm cover works at d = 450 mm, and every millimetre of extra cover is a millimetre of lost effective depth.
On the serviceability side, cover is the first term in the Eurocode 2 crack-spacing formula: sr,max = 3.4c + k1·k2·k4·φ/ρp,eff, so the maximum crack spacing — and with it the predicted crack width wk = sr,max·(εsm − εcm) — grows directly with the clear cover c. In the worked crack-width example, 35 mm of cover contributes 3.4 × 35 = 119 mm of the 335 mm crack spacing before the bar-diameter term adds the rest.
The two roles pull against each other: generous cover protects the steel but stretches crack spacing and thins the working depth, which is why cover is a specified, checked dimension rather than a construction leftover. Confinement calculations use it too — in development-length checks, cb is taken as the smaller of the side cover and half the clear bar spacing.