Hey Sanchari, we provide input elements to the "Start" Stage of each action page because Inputs to the Start Stage provide the means for a Process to supply data to the Business Object. That means you are providing details to the business object, with what details and inputs, it has to perform specific actions. In layman terms, its like providing fuel to Start an Action or process.
And yes you have to provide all the elements as input to the start stage on action page.