What is link building supposed to be?
It's mostly to give your domain more authority (thus making it rank higher) by putting your link on various other sites. Of course in reality, you want your link to be on other sites with relevant content.
They usually offer to post links for a couple of days, and sure enough, right on the dot, you can see traffic flowing in, once the period is over, it suddenly stops. Of course all traffic came from AWS or Google Cloud. I confronted one of the guys with the data I saw, he then decided to just keep one of his virtual machines up, and drive fake traffic to my site for months.
I did have some good results with some tedious data scraping tasks. But you have to explicitly tell them what you want, and in what format.