In the case of cost per mille/click, the publisher is not concerned about whether a visitor is a member of the audience that the advertiser tries to attract and is able to convert, because at this point the publisher has already earned his commission. This leaves the greater, and, in case of cost per mille, the full risk and loss (if the visitor cannot be converted) to the advertiser.
There are more than half a billion active websites under the sun and most of the (almost 70%) of them struggle to get traffic due to the high competition and expense of the process like SEO or Digital Marketing. However, you can simple buy targeted traffic from various sources at any time you want! Every paid ad campaign is an attempt to get traffic in exchange of money. However the reasons behind getting traffic are pretty different from each other. Some people buy traffic to get sales, some buy traffic to get leads, some buy traffic for brand exposure, some buy traffic to gain authority and improve rank on several metrics and so more.