There are two main reasons why this is the case. First, professional services firms can target the exact search terms that their target audiences are looking for in Google. As a result, a successful SEO campaign can result in a channel that continuously generates high quality leads for the firm.
Second, most people often start with the search engines to find information relevant to their interests or problems. As a result, they tend to trust the top results in the search engines without giving it much though. Getting the company website to the top rankings means that firms will be able to leverage the trust and credibility the users place on the search results. Getting multiple top positions also translates into increased visibility and branding.