Web developers need to discover how their clients define success for their web site. Sometimes the client can and will tell you up front what the site should accomplish. There are many factors involved in the success of a web site. But a surprising number of potential clients don't have a clear purpose. There are no right or wrongs in setting purpose, except not setting one.
Purpose can be practically anything.
Sometimes purpose covers several areas, and sometimes the only purpose for a web site is to experiment with online marketing techniques. Whatever the purpose, the customer must have a purpose, and know what it is, or their site can never meet their expectations. Audience Audience, for a web site, is similar to target market in traditional advertising. One web site can have multiple target audiences - for instance, customers and potential customers. In that case, the web site would need to offer content targeted to each audience.