Benefits of code reviews:

• Improves software quality.
• Reduces code complexity.
• Allows questioning design decisions while avoiding outright criticizing. For example, including React to a web project mid-way development.
• Share knowledge of the code base between developers. Reviewer can ask questions about the code. This educates developers about approaches new to them.
• Increases communication between team members.
• Finds bugs before they go live.

Good guidelines when conducting code reviews:

• Code reviews can be done by anyone, regardless of their level of experience.
• Code reviews should not require you to break someone's flow, reviews should be asynchronous. Submit a pull request and continue working without expecting another developer to review it right away.
• Reviewer, author and each commenter must agree on the review outcome.
• Pick a coding style for your team and stick to it.

All changes should go through a code review. But the following can be done without a code review:

• Removing commented out code.
• Formatting indentation or white-space.
• Removing dead/unreachable code, proven by a static code analysis tool.

Sources

• Introducing code reviews to a team – Part 1