Each SE site has its own Meta site.
Why isn't there just one common Meta site? What kind of questions are supposed to appear on the different Meta sites?
The per-site metas are for discussion of things that related only to their own 'main' site, while this 'main meta' is for things that apply globally (as well as for Stack Overflow itself).
If you wanted to discuss the closing/deleting of a question on a site, for example, you wouldn't post it here unless it was from Stack Overflow; you would post it on its own Meta.
Note as per nhinkle in a comment below, it is OK to post feature requests on a per-site Meta, as well. I would even say that it's OK to post almost anything Meta on your favorite per-site Meta most of the time. Sometimes, such posts will be moved here from their per-site, if they are deemed to be of general applicability.
There is some overlap in the applicability, then. But the general principle is that per-site metas are primarily there in order to facilitate local Meta discussion.
You can post any topic on a site's meta site that is of interest to users of that community.
This includes both specific topics only relevant there (discussing site/content policy, etc.), as well as topic relevant to multiple sites, like bug or feature-request topics. The team sees them and responds to them anyway. You can also post your support questions on how to use these sites there.
If you're not willing to participate on MSO if you're only e.g. on Super User, Gaming, or Cooking, you don't need to post your issues here. Just don't be surprised if you get a link back to MSO as an answer, when something's already been or being discussed here.
One could view the site-specific Meta sites as having the same purpose as local government in many governmental systems. That is to say that while it makes sense for there to be a large, centralized body (meta.stackoverflow.com) to address issues that effect everyone - how the system works, changes that will occur across the board, etc. there are many issues that only really effect the local users of a site.
Arguably, putting those questions up on the much more heavily trafficked main meta site means you lose the voice of local users.
Does CrossValidated really need dozens or hundreds of people who don't care about statistics weighing in on the CV journal club, or whether or not CV is appropriate to use as part of a statistics coursework? Or does Ubuntu or Bicycles really need me giving them my 5 cents, when I use neither Ubuntu nor a bike?
Decentralized problems are often best solved by decentralized groups. It allows sites to build themselves, support their own community, and defends them a bit from becoming "What StackOverflow users thing X site should look like" rather than "What X site's users do".