Accounting firms have a relationship with their clients that is pretty unique - you'll find a similar pattern in only a handful of other professions. It's a working partnership tempered with a high degree of trust, and often goes on for a long period of time. Because an accounting firm is built on these long-term, trusting relationships with clients, there's also a whole lot to say for how much of an impact customer referrals have on the business. Often, these referrals are one of the chief ways that a firm will bring in more business.
This pattern is functional, but it has its limitations. While customer referrals are a powerful way to bring in more clients, relying on it as the primary method of lead generation will ultimately restrict a business' growth, since the firm would only be bringing in a small pool of clients from a local area.