Presumably to get a true sense of the quality of the restaurants food and service.

If you book weeks in advance, they may prepare things differently (in a more costly/time consuming way) or have more staff on site, or make sure that the place is cleaner/tidier than it might normally be.

On top of that, if they don't make it clear that they're reviewers then it'll be an even more authentic experience.

You explanation make sense if the OP said 'unannounced', but they said the reviewers show up announced. That makes it sound like the restaurants know they are coming, hence the 'why' question.

My bad, that was a typo. They show up unannounced.

Spot on. I had a typo in my original comment, but they do indeed show up unannounced for the reasons you stated.

Hah, turns out two wrongs do make a right!

Sorry that was a typo. They show up UNannounced.

Cool. I’d love to see you in Boston!