I get where you're coming from, but if a friend asks me to look over an important email that they're planning on sending to their boss, I don't think of it as "wasting my time" if the email has a bunch of grammatical errors in it. Instead, I'm happy that I'm able to help them by applying a fresh pair of eyes to the problem, since they've probably spent a really long time working on the actual content of the email and haven't had the opportunity to step back and consider . It's just an efficient division of labor. In the same way, code review is a great opportunity to let someone who
wasn't involved in the actual writing of the code approach the problem & solution space with fresh eyes, now that you've done the hard work of translating the solution into something that actually works.
And code reviews are even more collaborative then that. When I'm looking over my friend's email, it's still ultimately their email and there are types of feedback that it would be inappropriate to give. But when you're contributing to a shared codebase, it's more like a novel that's been co-written by multiple different authors—obviously you want everybody on the team to be able to have input and feedback into what the "collective style" of the project is going to be.
