I was thinking in situations where a coworker might send me something to review, and I might have thought "hmm, I wouldn't have done it like that, but this is a great way to do it too". Also, a good source of teachable code is to participate in a programming contest, and then review the repositories of the teams who scored better than me after the contest.

I agree that if I don't already know how to implement something, seeing a solution before trying it myself is not great, that's like skipping the homework exercises and copying straight from the answer books.