From time to time you only want to run one test, one test method, one class or one package from the command-line. Or on the contrary: you want to exclude / ignore one specific test or group of tests during the build cycle. Excluding tests from the build cycle by the command line usually occurs when the following scenarios meet:
A test requires significant amount of resources (time, memory, disk space, etc.)
The run needs to be independent from the IDE (reenact the Continuous Integration / Continuous Delivery pipeline) as some IDEs load test-dependencies on the compile-time class-path.
You have no or limited ability to change the code-base