A link is a connection from one web resource to another, as stated from the (W3C).
It seems simple but web is done by millions of web pages, files, content of various types, and all of these take the form of two things: content and links, many links. A link building campaign is useful to increase back links to a website and its traffic, improving its rankings.
These links allow users to move from one web page the other, a simple click of the mouse can take you from one page to another, from a search result for a particular page, a video, a photo, or a song. If you don't have a website, a link would have no burden to you, but if you have a website, the links have a different meaning. In fact, the links will help determine the success of your website, because each "bot" (the search engine) makes an analysis of the links when determining the organic ranking (SEO) of your website in the SERPs.
For what reason? Think of a link as a vote given to your site, the more votes you have, the more authoritative it appears in the eyes of the spider. It is important to understand that it is better to have a few quality links that millions of links from sites that have no values (low PR). Why should your site be linked? What makes a web master to link your web site? The fundamental principle, what Sir Tim Berners Lee (the inventor of the first web page) had in mind, was to allow each document to bind to and receive links to high online documents. The link is represented by any type of link to another page, whether it's a banner, a text, a button, a badge, icon, etc.. To answer the original question: the lower the quality of your content, the lower the probability of receiving links. Often link building specialists, design link building campaigns, using their own tools, and their own strengths. As soon as the campaign ends, even the power of the link fades, this is because many web masters and bloggers stop linking towards the website to place. The effect of the link building campaign, decreases even more drastically when you buy links for a certain period and then stop. The design of a link building campaign (or a series of link building campaigns) requires you to be very prudent in assessing the strengths and weaknesses of a website.