Use case diagrams in UML are used during the requirements phase of software development – they usually put the system requirements into a diagram format, and it’s easy to see what actions a system must support at a glance. Of course, there will be many use cases for your system, and therefore you will usually need to draw many use case diagrams!
Along with other UML diagrams, such as activity, sequence and component diagrams, use case diagrams help you to visualize your software and requirements, before jumping in and starting to program.
Enable the UML shape library
In draw.io, all the shapes you need for use case diagrams are in the UML shape library. Click More Shapes … at the bottom of the left-hand panel, then enable the UML shape library and click Apply.