Automation testing helps in reducing regression test cycle time. For example, if the manual testing team requires 3 days to execute the regression test suite but the automation suite require only 5 hours, then that's a saving of 2 days per regression cycle.
Software test automation helps in finding bugs early and often. Manual testing may tend to overlook certain areas due to repetition. But automation suite executes all the workflow every time. This increases testers confidence and helps in certifying the build early.
When the deadline approaches, the QA team would be rushed to execute the test cases. There are a lot of chances to leave executing a few test cases. However, if your automation suite is integrated into a CI/CD pipeline, then the suite is executed almost daily.
Selenium can be used to perform cross-browser testing. This reduces the testing effort tremendously (at least in the order of 4).
In summary, automation testing helps in reducing the time for regression cycle, helps in finding bugs early and often so that the developers can fix the bug early and your time to market would be reduced.