Although it is true that the recruiter will often be taking a cut from what could have been given to you, it's not always true that this is the case. Sometimes the hospital would just be willing to pay a recruiter an additional fee, but not be interested in giving that amount to you. This may not be logical, but it may just be how the hospital operates. Sometimes you can even get a better rate going through a recruiter than directly through the hospital on your own...although this is likely a rare happenstance.
Hospitals don't always act in their best interest or intelligently. At my hospital, they seem quite willing to pay locums people top dollar instead of paying just a bit extra to their own staff, which would of course improve retention and decrease the need for locums people.