Offer services that compliment your legal service offerings -- Firms with ancillary businesses include those from an array of businesses, ranging from environmental consulting to risk management and human resources outsourcing.
Capture a greater market share -- Get more clients for both the firm and the ancillary business.
Be able to price services differently -- Law firms are typically tied in to the billable hour, but an ancillary business provides a chance to offer fees and rates that are more in line with the wishes of many clients these days (flat fees, project fees, success-based fees, etc.)
Employ a different business model -- Ancillary businesses allow firms to get away from the partnership system and could (and likely should) be run like a corporate business.
Invoke entrepreneurship -- As Charles Darwin said, "It's not the strongest of the species who survive, nor the most intelligent, but the ones most responsive to change." Any business in this day and age needs to embrace and support the entrepreneurial spirit -- it's what will separate the strong from the weak.