Joseph,
If a client admits guilt to his attorney, the attorney cannot put him on the stand if the attorney knows the client intends to lie. The attorney also can not plead the client innocent and present an alternative theory of the case. The attorney can plead innocent because of mental defect, diminished capacity, etc. and attempt to win the case that way.
That’s why the attorney doesn’t want to know. If an attorney representing Joe Schmoe in a murder trial can raise enough reasonable doubt by showing the deceased’s widow, business partner, competitor, etc. had means, motive and opportunity it might get Joe off. But if Joe has told the attorney he did it, then only innocent because of XXX can be plead and argued by an ethical attoirney.