A component job is the actual maintenance routine, which is based on a standard job description. The component job inherits all the elements of the standard job description, but you will be able to add a local job description to it in addition to a job interval.
The main advantage of having a connection between a standard job description and a component job is that making a change in the standard job description, will update all connected component jobs. For example, if you have 200 different component jobs in your fleet connected to one standard job description for megger testing, you can then make a change in the job description of the single standard job, and all of the component jobs will inherit that change.
This will save you going through 200 component jobs and making the change on every single one.