Delegate controls question

I understood that you can acomplish things like add a javascript to a page using that but im strugling to find an explanation of the diferences and advantages of delegate controls.

1. Do you need to override an existing one or can you create a brand new?

2. so, the advantage is the fact it is deployed through a feature, you can turn it on an off in a scope without editing code

so, is the advantage give non developer users from simple to complex functionality that can be activated without coding?