Despite the daunting name, pre-application advice is often just an informal chat with a planning officer before submitting a planning application. Some may charge for this service but that is at their own discretion.
It is always encouraged in order to see how local planning policies relate to your project! It also reduced the risk of an application being invalid.
Plans for the project must be complete before this stage in order to fully discuss the works that will be undertaken. It also gives planning officers a chance to see any potential site problems, before they arise.
Pre-application advice is also a way of ensuring that you can argue why your proposal should go ahead.