Obviously they have scads of info you don't, so they can theoretically help. If you want to ask for non-id or for search, it may very well cost you. My agency used to do this for free. They now charge based on a sliding-fee scale based on income. Many Catholic Charities will search too.
Fees vary tremendously.
Usually the justification is the hassle of file searching and then locating. It can be extremely frustrating for a birthparent or adoptee to know this person has the info right there, and they cannot find. One other thing you should know is that some agencies who are doing searches on behalf of birthparents have an internal policy of contacting the adoptive parents, not the adoptee. Probably they think they are being professional by dealing with their former client. They sure aren't giving any dignity to an adult adoptee.