Freshly poured concrete that has not cured can be damaged by rainfall if not covered. In a worst-case scenario, the rain washes cement out of the concrete, causing the surface to disintegrate into dust.
The removal of the surface layer erodes the ability of the subsurface concrete to maintain cohesion. Additional water can be absorbed this way, and the freeze-thaw cycles between seasons are more likely to crack the concrete. Sufficiently soaked new concrete scales and crumbles, essentially destroying the slab in layers. Generally, the susceptibility of concrete to rain leads to it being covered immediately after pouring, allowing the surface material a weeklong process of hardening.