As they search for ways to expand their legal services business inside the U.S., there are good reasons for the major accounting firms to focus on immigration law.

PwC on Monday became the second Big Four firm in four months to announce an alliance with a U.S.-based immigration law firm, following a similar deal by Deloitte in June. Facilitating global mobility is becoming an increasingly important business offering for these massive companies, and immigration law is a key component.