Execute SQL Server Agent Job task does exactly what it says: it allows you to run one (and only one) predefined SQL Server Agent job as part of a Maintenance Plan created with the Maintenance Plan Wizard.
Why would you want to run a SQL Server Agent job as part of a Maintenance Plan? In most cases, you probably wouldn't. As I have mentioned before, the biggest benefit of using the Maintenance Plan Wizard is simplicity, and if its simple maintenance model doesn't work for your environment, then you are probably often better off using T-SQL or PowerShell scripts to perform maintenance tasks, as they are more powerful and flexible than what a single SQL Server Agent job can do for you.
However, if you ...