Because the content is very interesting
When people find very informative or interesting content they are more likely to link to it to benefit their readers.
To get a back link
Many people link to other sites in order to get the chance of getting a backlink from the site they linked to. A webmaster can link to a site then notify the webmaster of the other site hoping to get a backlink.
Dissatisfied customers link back
Dissatisfied and unhappy customers can link back to sites that gave them poor service for the sake of bad mouthing the site. Those people usually write complains in their blogs then link to the website they are talking about.
They believe its an authoritative website
Authoritative websites usually get more backlinks than other sites. People usually link to sites when they believe they contain factual and authoritative information.
Link to a news source
Websites that publish news usually link to news sources. Sometimes webmasters link to sites that publish the news even if they weren’t the original news publishers. It all depends on which news source the webmaster will encounter first.
To give their audience more information
Sometimes webmasters link to sites that can give their audience more information about something. Instead of copying the whole thing webmasters provide links to sites that contain more information.
The content supports their point of view
People usually link to sites that support their points of view. When a person writes about a controversial topic then finds someone else writing about it he might link to that person’s site to get more support and to sound more authentic. (See Why Amp pages are good for your site?)
They are reputable sites
Some webmasters only link to reputable websites because they fear losing some of their page rank or getting a penalty if they linked to an unknown sites. Some search engines like Google put penalties for sites that link to bad neighbors.
It has a useful tool
People link to sites that have useful online tools. For example a tool that can determine site speed or assess the number of back links is very likely to get links from other sites if it was useful.
For SEO reasons
Some Webmasters believe that linking to good sites and providing references can help them with SEO (Search engine optimization). (See Why is SEO important to a business?)
To provide a refrence
People can link to sites in case they want to include a reference in their posts. People who copy facts that are new to them from other sites are more likely to link to them for referencing purposes.
Because they are not sure about the topic
When people are not 100% sure about the topic they are writing about they might link to other sites that contain proven facts to back their writing.
Why do people link to other sites?
Why are external links important? How do links to your site help you?