Andy Mallon explains why SSISDB exists:
Back in my day, we stored SQL Server Integration Services (SSIS) packages in msdb, or the file system. Well, I’m neither dead nor retired, so that’s not an accurate statement–today is still “my time.” But when I was learning about SQL Server, those were the only two options. If you or your environment predate SQL Server 2012, then you might only be familiar with those two options.
In SQL Server 2012, Microsoft introduced the SSISDB. Since SQL Server Integration Services (SSIS) is its own service, it deserves its own database. In a service-oriented world, it is discouraged to put data for one service (the database engine) in the same database as another service (integration services).
Click through to see where it comes from and when you might need it.