The beam, which is reinforced from both zones i.e., compression and tension, that beam is said as Doubly Reinforced beam. It is usually given when the depth of the beam is constricted. It is obviously not safe to provide a singly r/f beam with min depth resulting in failure due to insufficient resistance to oppose the bending moment.
Moment of resistance could not be increased by increasing the steel in the tension zone. Rather it can be increased by increasing the reinforcement, but it should not exceed 25% of the strained side. That’s why a doubly reinforced beam is provided to increase the moment of resistance of a beam having limited dimensions.