The description of a use case allows you to:
*clarify the course of the functionality;
*describe the chronology of the actions to be carried out;
*to identify redundant parts to deduce more precise use cases that will be used by inclusion, extension or generalization / specialization. And yes, in this case we will be iterating on the use case diagrams;
*to indicate any constraints already known and which the developers will have to take into account when making the software. These constraints can be of various nature.