The advantage is that /code-review supplies a structured idea of how to review and what that process should look like and then launches independent subagents to approach the issue from multiple angles.

It's analogous to how in the early days you could see benefits by telling the models to "think step by step". /code-review is something like "review angle by angle". "Consider removed behavior" and also "Look at language gotchas" and also "Look at test changes"...etc. Yes these are all somewhat implicitly already part of what "code review" means, but the models perform best with explicitness.

If you want my 2c as a power user: just don't think about it and use /code-review xhigh --fix. This will cover like 98% of what you want out of code review. It's a good skill.