Joe Gavin prunes some history:
The SQL Server Agent is a very powerful job scheduling and alerting tool that’s tightly integrated with SQL Server. It’s quite possible you’re only using it for basic maintenance tasks like database backups, index maintenance, DBCC checks, etc., and the default retention is fine. However, you may also be using it for much more, i.e., executing multiple step jobs that execute multiple SSIS packages, and you need to retain a longer than the default job history.
Read on to learn more, including how one really useful-sounding checkbox doesn’t quite work the way you’d expect.