In going through the first post queue I came across this post which was posted by a now-deleted user. It would seem to me that most of the reasoning behind the queue is to help out and provide feedback to a brand new user, which can't happen if that user no longer has an account on the site.
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
- Your voice helps shape the way Stack Exchange operates
The primary purpose of the First Posts queue is the same as with the other queues, to review content. Providing feedback to a brand new user is important, but you can still review the answer even if the answerer's account is no more:
I realize that #3 might sound strange, but your comment will be read by everyone who happens upon the answer, not just the OP, if you have something useful to say, say it.