This is the internet. There are all sorts of people here, good people, bad people, trolls, haters, yaddayaddayadda. As you've probably seen, not all of what people post is helpful to others or this website. In the past, I've seen people posting an answer only to mock the QA, without actually bothering to give any advice. I don't think those kind of behavior should be tolerated, because people came to this website for help, and these people bully them instead of giving advice. Another reason is because of the spamming. Just like any other forums or websites, this website isn't free of spammers. Would you want to see tons of spamming "questions"? I bet no GAG user needs and wants to see those kind of posts. So the admins delete them, and moderators help the admins catch them. There aren't that many admins, so it's impossible for them to see everything. That's why they need moderators to help "hide" the bad posts first to be permanently removed later, or to be put back up on the site if it doesn't really go against the rules.
If you have read the FAQ and know the guidelines or can behave according to the normal social conduct as to not offend other people, then great! More power for you! But unfortunately, there are many who can't. Many new users don't read the FAQ and asks questions that offend certain groups of people, harrass other users personally, and spam.
Conclusion: admins and mods are supposed to help make this site as pleasant and helpful for everyone as it is possible. A site where spamming, bullying, and offensive posts aren't punished or removed is not a pleasant site.