Selenium provides comprehensive low-level functionality for interacting with browsers and web elements. Beyond test implementation, additional tasks are required when it comes to management, such as test data management, result analysis, execution management, as well as the continuous adaptation of the test cases.
Eclipse installable units are what you publish to make your Eclipse features and plugins available to your users. You might also want to provide your users with custom Eclipse distributions or products where all your stuff is already installed.
Hopefully, you have lots of tests for your Eclipse plugins, both on the core level (unit tests) and on the UI level (functional tests).