The "committee" interviews each pre-med. For example, I interviewed with two advisors, one of them being the department chair for the Honors College. They ask hard questions about your application to prepare you for the real thing.
But it's not just practice, because they write a summary letter regarding your application and append it to your letters of recommendation. You do not get to read the summary letter, and the summary is, from what I understand, quite frank. If you're "middle of the pack" relative to the other Pre-Meds from your school, they say as much. If you're not as good an applicant, they say as much.
Generally, medical schools expect to receive your letters of recommendation from your pre-med committee, if it is known to exist. The committee will not send your letters of recommendation without conducting an interview and writing their own letter. Thus, it is basically a required additional step.
Bypassing the committee by using a third party service is frowned upon by nearby schools that have acknowledged the credibility of the pre-med committee.
For example, here at Mizzou, if you bypass the committee, you can expect Mizzou, SLU and Wash U to notice since they get so many applicants from here and they acknowledge the value of the committee letter.