This is a great and common question we get from our clients at
Admissionado.
It obviously does not hurt if you have an alumnus write a letter of recommendation, but it will have a marginal benefit at best. To be blunt, the only way it might have an outsize effect is if the recommender happens to be an influential alumnus and/or a big donor.
Assuming he or she is just a "regular" alumnus, one of the indirect ways it helps, is that the recommender may be able to make the "fit" case more effectively by tying the skills and experiences of the candidate to the specific culture at his or her alma mater.
toDoorNot is also right. The most tangible way we have seen alumni help is if a candidate finds themselves on the waitlist. The best "extra" information an AdCom can receive besides a higher test score (assuming that was in play) is a letter from an alumnus.
Hope this helps! In case anyone is curious, at
Admissionado, we offer services for both letters of recommendation and waitlist support. Reach out if you are interested.
https://admissionado.com/Best,
Seth