Leaderboards are ordered by one Statistic name. In some cases, that value can be same for a few players. Adding secondary value can help them to sort. Even third value can be added.
Example: Kill count of 5 players can be same, let's say 30. In this case, we can look for how many item they looted, or how short was the ending the level, to sort them out, instead of saying all 5 is has the fist place.
Sorting them by name or id is not that much fair :/