I flagged this headphone repair question that had no schematic/pictures as "off topic".
I certainly do not wish to use the flag system improperly.
Why is that a bad use of the flag system, and what would be a "good" use?
Help center says, for the flagging privilege (obtained at 15 rep):
When should I flag?
If anything happens on our site that makes you feel uncomfortable or that, in your opinion, clearly does not belong here, please flag it and bring it to our attention!
You saw something that, in your opinion, clearly did not belong here, and you flagged. Fine.
What happens when you "flag to close" before having the close privilege is that the question goes to the close queue, and users with sufficient privileges will then cast their votes according to what they think (but the flag does not count as a vote).
Nothing wrong here.
On the other hand, most of the time, other users with sufficient rep will likely close the post by themselves anyway, and the result will be the same. But in that case, the fact you flagged does not induce any overhead on the higher-rep users, so it doesn't matter. There is an overhead only if you are the only one who thinks this post must be closed (but you don't flag wrongly, do you?).
In addition to the other answers, moderators can take some flak when community moderation isn't allowed to take it's course before they zoom in and close something. It takes 5 users to close a question, but just one moderator. They tend to reserve that action for cases that might fall between the cracks, or for cases where there is a reason to get something closed FAST (like site sabotage, profanity,...)
The places where flags are most useful are for things that won't work their way onto the close cue, like questions posted as answers to similar questions, answers that are really comments, etc.
Flags are meant for signaling exception conditions to the moderators.
Closing a post is routine business. You only need 3000 rep to be able to vote to close questions. We have a reasonably large number of users at 3000 or higher, so bad questions will get closed soon enough. If you don't have 3000 rep yet, just move on and let someone else handle it.
Early in the site's history, there were few users that could vote to close. Back then, flagging a question to be closed made some sense. It no longer does today.
The privilege of flagging to close is still there. It's not wrong to use it, but also not necessary and now how the site operates today. If you see a question that should be closed, just move along and let others handle it until you have the necessary rep. You could still leave a message telling the OP what is wrong with the question, and warn him that it will likely be closed if he doesn't attend to it soon.