Trust is your currency. Most people hiring a lawyer are doing it for the first
time, or doing it again after it went badly, and they have no way to judge the
quality of your work before they call. So they judge the thing they can see: what
other people said about you.
Social proof does the persuading here. It raises trust, it raises revenue, and it
lowers the risk of the one angry review sitting unanswered at the top of your
profile — which is the version of this that costs firms real cases.
The system has to be passive. A review program that depends on someone
remembering to ask stops working the first week the firm gets busy, which is
exactly the week you most want it running.