Understanding your demographic is important. Creating a user persona (or multiple) within a web build allows us to plan user journeys for each person.
Generally these personas will fit into stereotypical states, the personas will be general and generic of a type of customer/client that uses the site, this could range from those less tech savvy to those who understand the ins and outs of how to use a website.
The personas will be representative of a major group of your website users, it will express and focus on the major needs and expectations of these user groups, aid in covering features and functionality and describe real people with backgrounds, goals and values. We will take the time to understand these different users, and what they like/dislike about your website, and what improvements to make based on our findings.
The reason why we take this path is because it allows use to evaluate new site feature ideas, inform us of information architecture to develop through wireframes, behaviours and labels, inform the designers of how to create the overall website look and feel and the rest of the team of what should be integrated into the site when it is built to aid in the best experience for each user.