schools this for two reasons
1) to an adcom, a premed committee evaluation is only as good as the history of candidates they have previously sent letters on. That is, if a premed committee writes support letter for every mediocre applicant and that reputation is known by an adcom, their letters have little value to the adcom, which goes along with comments from
@Angus Avagadro and
@Moko This of course, makes letters from UG premed committees with good reputations, like those with high standards for GPA, quite valuable and impactful to an adcom.
2) The undergraduate marketplace is a competitive business. If a school controls the premed process via strict guidelines for committee letters and only supports strong candidates, who are more likely to be successful, then the school can boast and market the success of their premed program.
3) Also to note, there is no requirement whatsoever that premed committee letter includes the underlying LORs in their packet. A few schools do note include them. And I know more than a few schools that do not collect them at all. They instead have the applicant provide a list of professors who is then contacted by the advisor.
4) This brings me to my last point,
there is no set of standards for premed committees except general guidelines. Some schools like Rice and I believe still Cornell have very strict, rigorous process than essentially make you effectively write both your PS and multiple likely secondary essays and prepare applicants extremely well for the process. Other committees and advisors seem to have little or no useful process.
