Doubly reinforced beams are important when we have a bending moment which is generated in them. Generally, this reinforcement is used to withstand the tension. Hence, when the size of the beam is constrained, you should pick these reinforced beams instead of the singly reinforced beams. Also, in beams you can see more than one conditions, such as the continuous condition, the supported condition, the cantilever or else. All these elements can cause bigger bending moment which needs more than the singly reinforced beams.
Doubly Reinforced Beams and their Importance
Finally, you can watch the tutorial below. Also, for more information, you can click the link and learn something more about this topic.