I'm sure it's happened to you: you're browsing Stack Overflow, when you see that amazingly tempting HNQ in the corner. You click on it, and have an immediate urge to comment/upvote. You click the little button...and see the "Join this Community to post" popup.
Of course, you can just join that community, but you'll eventually end up with tons of Stack Exchange accounts with no questions, answers, or reputation above 101. It would be great if you could just click the dang upvote button! Of course, as a user without the starting bonus of 100, this shouldn't apply (Except maybe for comments).
At first, this sounds kind of dumb. But when you see a good question, or a great answer, or have a sudden urge to comment, you should really be able to!
Edit: This post has been marked a duplicate of this question. It is clearly not, because my question is from a completely different viewpoint. They are anti-duplicates.