An endpoint snitch that automatically and dynamically infers "distance" to other machines without having to explicitly configure rack and datacenter positions solves two problems:
The killer feature here is adapting to things like compaction or a failing-but-not-yet-dead disk. This is important, since when we are doing reads we pick the "closest" replica for actually reading data from (and only read md5s from other replicas). This means that if the closest replica by network topology is temporarily slow due to compaction (for instance), we'll have to block for its reply even if we get the other replies much much faster.
Not having to manually re-sync your configuration with your network topology when changes (adding machines) are made is a nice bonus.