Sometimes unboxing can give you valuable and otherwise hard to find information, e.g. how big the transformer is and how it looks. That stuff is typically nowhere to be found, not on the website of whoever made the thing (even if you search for the manual) and often not in reviews (at least if the transformer isn’t extremely enormous or extremely ugly or otherwise noteworthy).
First impressions tend to be pretty useless in my opinion since it’s not very thought out and all very subjective. Maybe sometimes something useful is said, but it’s mostly useless.
It’s also good for getting a non-product-photo look at the product. For some things there just aren’t any video reviews and photos often just don’t cut it. It’s always good to get more than one look at a product.