Well it depends on where the traffic is coming from. If you get some reputable
SEO company to build links and place ads for you, then you will get qualified traffic, which will get you conversions and increase your rankings as well. However if you read something promising you tons of traffic and all kinds of miracles, I would be suspicious. Not only they might send you crappy traffic but if they are shady, chances are that they have already been flagged by Google and their traffic might in this case hurt your rankings.