Back in the day, Google determined relevancy of an article mostly based on keyword density. In other words, the more you use a certain keyword in an article the better your rankings are. This method came to be known as keyword stuffing. As you can imagine, such SEO strategy led to the creation of some pieces of content that were virtually unreadable by a human eye, not to mention they were completely useless.
Of course, given that Google search engine is primarily concerned with finding the best content which can provide valuable information to the end user, keyword stuffing became quite a problem. So, in order to remedy the issue, Google started implementing LSI keywords instead.