To reduce the number of unknowns in solving the problems and when it is enough to get efforts along the bar without information about displacements. In other words, you will get efforts and displacements in places where nodes are located. And in design sections you will get only efforts without information about displacements.
For instance, if you would like to add reinforcement to the beam using one finite element then you will miss reinforcement in span. Since bar has two sections by the default – at the beginning and at the end of the bar. If you use detailed grid of the FE then you don’t need to add design sections. One more example is set of the load along the length of the bar. In order not to miss changing of efforts along the length of the bar, the design sections are also added.
It is necessary to set design sections adequately because their amount will affect the calculation time of the DCL and DCF as well as the calculation of the construction.