A permit to work system will help to ensure that all work performed on site is a. Planned and approved b. Performed safely, and c. That all necessary personnel are informed.
As an example, imagine you run a food factory and one of your production lines needs some welding work - this seems straightforward but let's look at some of the things that may affect the safety/effectiveness of the task.
A permit to work system will help you ensure that all these questions are answered before work commences - it should also ensure that permits are easily managed from a central location and ideally with a visual representation of the work area - we call these 'Plot Plans'
Will working at heights or confined space working be involved? - what precautions will be required
How will other people know that the work is being done - so for example electrical work on the line isn't scheduled at the same time.