A brainstorm produces what the room thinks. Reviews record what the buyer felt, in the words they actually used, at the moment they cared enough to write them down. When we build creative strategy for a brand, we start by reading hundreds of them, the brand's own and its competitors', before anyone touches a brief.
What we are mining for is specific. The desire underneath the purchase, which is rarely the thing on the front of the pack. The moment that triggered the search. The objections that almost stopped the order. The exact phrases people reach for when they explain the product to a friend, because those phrases are hooks that arrive pre-tested.
In our experience the difference between an ad that tests well and an ad that dies in week one is rarely production quality. It is whether the opening line names something the customer already believes. You cannot invent that in a meeting room. You can only find it, and the reviews are where it lives.