Just as in business, politicians are selling a product. That product is themselves. They are selling their image, their views, sometimes even their appearance. They have a public persona that must be marketed to try to obtain the votes they need to achieve public office. Usually, they market themselves to various target populations. If they want to reach the working man, they will market their ads accordingly. If they want to reach out to the upper class, again, they will manipulate their ad campaign accordingly. Marketing is super important for politicians because their image is their brand which they are selling to the public.
I agree that politicians need to brand themselves. If you are asking about commercials, I would say that they do drive some people's opinions. Not everyone is going to watch the news, read the paper, or go to a candidate's web site. You can't help being bombarded by TV ads.
Politicians need marketing because they are trying to sell something (themselves and their plans) to voters. They need to reach out to specific groups of voters in ways that will appeal to those voters. If they don't think hard about how to do that (which is essentially what marketing is) they won't get the people to vote for them.