The front view is not always as bad as offscreen view. And sometime tester can find a problem only looking into 'other side' of the product, where can be hidden messy code. Why messy code is bad for the project? One big reason - because it is more likely to do mistakes in it. Second reason - because the other developer could not understand some crazy magic logic of the author.
The second reason is more important to me, than the first one. The bugs can be found and fixed, especially if place of the messy code is known, so you can pay special attention to it. But if you need to fix some bug as fast as possible and the developer of the messy code is on his vacation or being ill, at best it takes a lot of time for other developer to understand the mess. The worst case - other developer "will not better touch anything", because he can not guarantee not making other bugs.
So, if tester have spare-time, he can always do a code review.