An attorney is a “service”. That’s what this retainer bit is about.
It's more than that. It also begins the attorney-client privilege. You "retain" the services of the attorney so that he can't also become the attorney for someone else involved in the case, including potential witnesses. That would be a conflict-of-interest, so the concept of "retainer" is to keep the attorney from being the lawyer to multiple parties in the same case.
-PJ