This may sound materialistic however it is the fastest and most direct method to get unnecessary things out of the way. By knowing of the client’s intended budget for the site, the designer and client can immediately discuss realistic goals for the site. For the designer, the complexity and extent of the site is made clear. And for the clients, it helps eliminate a lot of modules such as recommended plugins that are not mandatory for the site to function (optional add-ons).
Setting a budget helps to filter options realistically, effectively and efficiently.