The chlorine atom in chlorobenzene is ortho and para director because

An connected chlorine atom has a dual nature.
Due to the resonance effect, chlorine atoms have an electron-releasing behaviour.
The chlorine atom's single pair delocalizes with help from the ring's pi-electrons.
Due to an electrophile attack, the electron release into the ring stabilises the positive charge at ortho and para locations. Therefore, ortho and para locations are preferred for the substitution of an electrophile.