Hasn’t it been a long-running complaint that admins can just edit karma and so forth however they want? By putting this on a blockchain it creates a public record that can’t be tampered with without public awareness, and depending on how they do it it can’t be easily tampered with at all.

Again depending on the technical details of how they implement this, the network fees may be trivial - small enough for Reddit to subsidize without the users having to worry about it at all.