I understand that md5sums help you know you got the same copy. Why not share his resource directly instead? Also, what if someone gets the same resource "Formal

I understand that md5sums help you know you got the same copy. Why not share his resource directly instead? Also, what if someone gets the same resource "Formal Languages in Logic: A Philosophical and Cognitive Analysis" under a different md5sum (for example, the format or edition is different)? It seemed somewhat useless in this case.