As part of its regular, ongoing, day-to-day activities, your SQL Server instance would naturally collect historical data about its automated operations. If left unchecked, this historical data could pile up, leading to wasted storage space, performance hits, and even worse issues.
MSDB would obviously be collecting data about the SQL Agent job executions. But there are also a few other types of historical data that needs to be cleaned up once in a while.
Click through for part 2 in the series, which covers a half-dozen more things.