"Purge replays" would be similar to "Mass Replay Check", but it would delete all replays that don't pass, and so it won't need to generate a text document summary. (It should also warn the player before starting work, just in case they click on it by accident.)
It's possible to achieve the same result by running Mass Replay Check and deleting replays manually, but that's more laborious and this would automate the process.
The main use case is when importing a pack and its associated replays from an older version of NL -- usually I have no use for replays that no longer work, since I will have to re-solve the levels anyway. Also, if I was in the middle of playing a pack, then I am hoping to finish it eventually and post replays for the pack author when I do, and then it's just a courtesy to them to exclude replays that don't work any more. (The only time I can think of that I might want to keep defunct replays is when it's a pack I'm designing, since I may want to know how people have solved my levels, and I might not have gotten round to looking at all replays and making appropriate changes by the time a new version of NL is released. Needless to say, I play many more packs than I design.)