You wouldn’t attempt to build a house without having a detailed set of plans; the same goes for an application development without first defining your functional requirements.
In a broad sense, requirements analysis is about defining and communicating a platform’s functions, such as what happens when a user buys a product through a shopping basket system or when a user submits a ‘contact us’ form. Devising a list of functional requirements will ensure that all parties are informed as to the expected outcomes of the website design.
In some cases these specifications take the form of graphic ‘wireframes’ or ‘blueprints’ that map out the content to appear on key pages of your site. In others, particularly involving complex custom-made designs, a comprehensive functional specification may be recommended to capture exactly how the system will operate.