The purpose of a website present a message, build trust, and encourage the viewer to take action.   

A website is a method for a business or individual to communicate their brand, message, product, service, and or ideals to their target audience. It should communicate, inform, and connect to the viewer, through imagery and text, presented in a user-friendly delivery system.

It's important to ask the right questions to establish a client's purpose and needs.

What a your goals for the website? (E-commerce, Informational, Service, Gallery or Portfolio, etc.)
Who is your target audience?
What are the outcomes you expect from your site?
What actions do you want the viewer to take?
What is the matrix of success for your business?