Originally, the review queues did not have a "No Action Needed" button. My understanding is the logic behind it was that every post reviewed deserved some action. Meaning, if there was nothing wrong with the post, then it was worthy of an upvote, but otherwise it needed an edit, comment, downvote, or any combination of the 3.
However, the "No Action" button was added for several reasons:
- Conscientious reviewers felt there were just some posts that did not need an edit or comment, but were not worthy of an upvote or downvote, and didn't want to use "Skip" since it doesn't give credit for the review.
- Robo-reviewers & badge hunters would upvote a post as it was the fastest way to review the post. This resulted in a lot of poor questions & answers getting unwarranted upvotes. (In addition, this kicks it out of the queue so it didn't get other, possibly valid, reviews)
I think the reasoning for the separate buttons is there is separate logic for it. "I'm Done" is supposed to signify that the reviewing is complete with the reviewer taking 1 or more actions on the post. "No Action Needed" was designed to give users an out to say "I don't need to do anything here, the post is fine as is". When you combine multiple functions into a single button, I think you run the risk of confusing some users.
Personally, I like the separate buttons. While you don't run into this much anymore as the First Posts/Late Answers queues do not get very large and you don't do a lot of these reviews en masse, the workflow is a lot easier when each button has a specific function and there are no "hidden" buttons that appear. Disabled buttons that enable after a period of time are much easier to work with.
Some related background: