Succession: Most smaller firms do not have a succession solution internally. For those firms, it is an obvious and important matter to secure your succession with an external firm.
Cross selling, growth. There are many niches out there and many smaller firms have clients that may be receptive to certain niches but lack the time, knowledge, capacity and, or licenses to deliver. Also larger firms may enable you to attract additional clients.
Back up, security. Many small practitioners fear the risks associated with short and long term disability, death, the desire to go away on a vacation and have someone watching the fort.