What was the significance of the leaderboard?
Most directly, of course, it was that the system used to distribute rewards. Even the club guilds couldn't disregard these extremely valuable rewards, so they too actively strove for the top spot.
Aside from this, the leaderboard was also a way of proving the strength of an individual or a guild. An in-game event like Ghost Parade was partially based on luck, but when the top of the leaderboard was entirely occupied by pro level accounts, it was clear that luck only mattered when skill levels were similar. In front of the crushing power of pro players, no matter how good an ordinary player's luck was, he simply couldn't compete with the raw skill of the pros.