I assume with enough accounts that look legitimate, they can shape overall "consensus" opinion on something, which would be valuable for all sorts of reasons. Some of those reasons being obvious (promoting a particular product or service) but others being more subtle ("manufacturing consent" for, say, a war in the middle east on behalf of some group)

We all like to think we're independent thinkers, but when seemingly everyone has an opinion a certain way... it would still, at least subconsciously, sway the average person.