Lawyers are trained to advise people about how to exercise their rights and to fulfill their obligations in accord with law. Ideally, lawyers help people to avoid legal mistakes in their actions which could prevent them from accomplishing their goals. The job of a canon lawyer is to see that the carefully devised rules of Church order are properly understood and applied. Use a canonist, then, to understand better what your basic (and not so basic) ecclesial rights and obligations are in the first place. A canonist can only work within the structure of the law of the Church, in so far as all parties abide by the law. This is important to remember when evaluating a canonist's performance, since more complex canonical issues may take a lot of time to sort out, especially in case of allegations against priests.
Indeed, canon law demands that parties try to settle their differences before resorting to formal canonical processes. But if one is unable to effect a resolution to a situation, or if one is actually the object of disciplinary action in the Church, such persons can, and should, make use of canon lawyer(s) to represent their interests in light of the ecclesial common good.