You usually get transmogs from old reps. Some of them have gear you can buy from rep vendor to get the transmog, while others sell tailoring, leatherworking or blacksmith recipes which you can craft and learn the transmog or sell it on AH. All of these are pretty good especially after 8.1 because Blizzard removed rep requirement to use transmogs from rep rewards on your other characters.
Some rep vendors like Wyrmrest Accord or Shatari Skyguard sell mounts at exalted rep as well, while newer ones like WoD, Legion and BfA rep vendors also sell toys and pets.
Besides, old reputations are still useful as it’ll add up towards 100 exalted reputations achievement and once you get there you’ll be rewarded with a nice mount.