Word of mouth is the most trusted form of marketing. Both your friends and influencers on social networks convince you to buy stuff.
But a room of anonymous people can also impact your purchase behavior.
Everyone's got a Suburban because it's the most ubiquitous car in the neigboorhood. Everyone's got a dog because the entire apartment does too.
Buying is fitting in. Comparison prompts the urge of fulfillment. People are built to copy other people.