[The next four posts are reasons why, if presented with a legal problem, you should acquire the services of a large firm.]
Expertise.
Big firms, with lots of lawyers, pay very well. Many big firms have lawyers who are considered to be among the best in their profession nationwide, not just in a small region. If you have a sufficiently complex (or expensive) problem, you may feel that you need the top person you can get- and that person is often found at a top firm.