10k users can dispute other users flags if they think the flag was in error. This has two major effect:
- The number of users that disputed a flag is visible to moderators
- The target flag is declared disputed, it doesn't affect flag weight either way, and the user will get the feedback that the flag was disputed in his flag history.
The first part is the useful part of disputing flags, as it provides the moderators some additional information, showing that a certain number of users disagree with that flag.
The second part is rather useless, and even actively misleading. The fact that the flag was disputed is not useful feedback, it only means that any user disagreed. Even if a dozen other users and a moderator agreed with the flag, it'll end up as disputed.
A user shouldn't even have to know that his flag was disputed, if the community or a moderator finally acted on it. The flag should be declared valid in this case.