Why? It allows you track visits on a specific web page. Whatever page that newly acquired customers land on after the sale, this will be your “Destination.” You can add the URL of that page in the “Goal details” section. There are other optional details that you could include. For example, you can assign a monetary value to a newly acquired customer. You can also map the journey that customers take up until they convert. At the very least, configure the first option. You need that hard dollar value to calculate ROI.
Black hat SEO refers to the practice of trying to trick the search engines into giving you higher rankings by using unethical tactics, such as buying links. The risk is just too great. Even if you enjoy a temporary boost in rankings due to black hat tactics, it’s likely to be short lived. Google is getting better and better at spotting dirty tricks and sooner or later the progress you made will be wiped out by an algorithm update, or worse, your site will get removed from the index altogether.
Regarding Link Detox, links it diagnoses as Toxic are generally fine as they're either not indexed by Google or have malware/viruses/etc., but I recommend a manual review of any diagnosed as Suspicious. I used it recently to get started cleaning up our backlinks and some legit sites and blogs were under Suspicious simply because they didn't have many links pointing to them.
Now, it is not that these sites are not interested in Google users. In fact, they have hired us to help them increase their share. However, they are getting so much traffic from sites like Facebook that it seems there is less urgency about attracting this traffic and less willingness to change the site to meet organic standards. Not long ago, sites would urgently and unquestioningly abide by Google’s standards to court that traffic.