The system for closing questions on an SE site is rather simple. There are various types of close reasons and votes, that fit in roughly three categories:
Low Quality Posts
The close reaons "unclear what you're asking", "primarily opinion-based" and "too broad" are intended for questions that, while potentially valuable and on-topic, currently do not fulfill our quality criteria. They may be hard to answer because information is missing, or the way they're asked does not work well with our format. These questions should be edited so they can be answered properly, or deleted. Once edited, the question can be reopened and answered.
"This does not belong on the site"
Primarily, this is about closure as Off-Topic. These are questions that, even if they fit all the quality criteria, don't belong here and are accompanied by a description as to why that is the case. Because of this, the close system also allows for custom close reasons under the Off-Topic label. Every site can define its own reasons for closing a question as Off-Topic, and every user casting a close vote can detail their own specific reason if the given ones are insufficient. These questions are meant to be deleted, they add little to no value to the site.
Duplicate closures are special: These questions are meant to stay. They are usually alternate ways of asking a question, and are therefore valuable for anyone else looking for the solution. Duplicates point at other questions, the duplicate targets, which should contain an answer to the question - and all of its duplicates. For unregistered users, duplicate questions even automatically redirect to the target.
That said, there are some pitfalls with duplicate questions: The same answer a duplicate does not make. Just because two questions have the same answer, doesn't mean the questions are actually the same. Duplicates are for good reason named exact duplicates: This should only be applied when the question itself is actually the same, and not just superficially.
Now, as for the issue we're facing:
Questions are not closed, or closed for the wrong reasons
As I said, each of these three categories has its own future route. Some can and should be fixed, some should be deleted. With the tools we have, closing a question early and properly is a very important thing. A closure shows the user why the question currently has faults and tells them what to do about it. If we close for the wrong reasons (such as an incorrect duplicate target, or Off-Topic instead of low quality), we make it harder for users to find the information they're looking for and we don't actually tell the users what they should have done instead.
Ideally, when a question is asked, it is either closed or answered before it vanishes off the front page. Of course some questions are harder to answer, or the experts aren't available sometimes, but this is the ideal state no less. If you have to ask for more information, cast a close vote as "unclear what you're asking" as well - it can still be reopened when the information was provided. If the question is opinion-based, try editing it and making it more objective, or cast a "primarily opinion-based" close vote. This goes for the other close reasons as well.
If we can get on top of our new questions and start closing them better, we can keep the quality of our site on a good level and make sure the site doesn't get cluttered with hundreds of identical questions, or questions that are bound to never get an answer because they just cannot be answered without speculation.