Definitely this. It can be helpful to both parties. The learning opportunities for experienced developers start to thin with time, and one of the best ways to combat that is to get a fresh perspective.
In my experience, even if you're not learning any new concepts or techniques, you're learning about parts of the code base you would maybe not touch normally. This can be a huge advantage when it comes to localising/fixing bugs later, or when the "owner" of some code is on holiday (or sick, or has left the company) and urgent changes need to be made in "their" code. Having at least 2 people know about every corner of the project is worth the cost of the code reviews.