Spinnaker: he doesn't want his leaks removed, he's asking what leaks actually do, why they're so bad.
Answer: You're avoiding lag (extreme lag).
I don't know if you're new to warcraft, or a veteran, but did you ever play a map called "Wintermaul Wars"? The lag there is caused by leaks.
If you haven't: in the middle, the map started to lag. As the game progressed even further, people were dropping out because they simply couldn't do anything, the lag was just too annoying (you could move your mouse once every 2-3 seconds or something, end-game you could do 1 action every 5 seconds).
Even the best PC's had a lot of trouble running that map.
There was only 1 reason for that extreme lag: memory leaks. There were so many of them that they completely ruined the gameplay due to the lag.
Nearly all spawn-based maps had this kind of lag a few years ago.
And even if it isn't a spawn-based map, other maps also had trouble (though not that extreme, still very noticeable).
Remove them and your map should run smoothly, don't remove them and your map will lag mid-/endgame.