Most ads aren't aimed at getting you to click on them and make a purchase. The goal of an ad is to put a concept in your mind. The purest form of this are the political yard signs. Nobody thinks, "I saw 90 signs for Bob and 110 signs for Mike; therefore I will vote for Mike." But people do respond to pressures like that; it creates biases and impressions that they don't even realize.
Even when you do your research, you are influenced by these. Most of the time, your research is going to be inconclusive. There isn't any "best chair"; at best, it's a matter of personal taste. Most products, from canned peaches to computers, will end up having similar specifications, but you'll have a preference because you like the flavor of this brand or the you had a good experience with that computer in the past.
Advertising helps put those ideas in your head. Just seeing it in the ads will give you a positive feeling toward the brand, if the advertising is well done. A lot of advertising is poorly done, of course, but a well-done ad can influence preferences in very subtle ways. That subtlety means it's aggravatingly hard to tell which ads work and which ads won't, but advertising continues to exist for a reason: it steers consumer preferences during the phase where they don't know what they want and end up trusting their instincts. Which applies to more purchasing decisions than most people realize.
Stupid advertisers want ads that they measure working by clicks, so they optimize the ads to attract clicks, but that doesn't drive purchasing. The best ads are the ones that consumers don't even realize they've seen, but just develop a cumulative effect of exposure. That's hard to do, and requires a lot of time, money, and effort to get right. Even then it's a crapshoot, like trying to write a popular song. But in the end, there's a market for so many chairs and so many peaches and so many computers, and advertising can steer enough purchases towards yours and away from somebody else's equivalent one in a way that merely improving the specifications can't do.