If you set the WHEN condition to be 'Started by action', you turn the entire behaviour into an action that can itself be triggered by a condition, either in another behaviour or in the same behaviour. It is to be used with the DO: Start behaviour action.
You can think of it as a nested behaviour which can be referred to by other behaviours, without having to write the same behaviour over and over again.
If you wish to create actions that loop, take a look at DO: Loop
Example
I wish for two different conditions to activate the same behaviour. One condition is states matching an order of switches, the other is a Variable reaching 5. Both will refer to the same behaviour in Start behaviour action.
