Basically , what it does is that it shows all those keywords that a particular website ranks for. By keywords , I mean all those search terms that people would type in to reach a particular website.
More good keywords would mean more traffic and ultimately more traffic would mean more money from the advertisements. So, if someone has a website and has a very hard time wondering what the next post update should be, then it is better to take the help of semrush. You can go to the websites that are similar to your niche and website, and get an idea as to what should be your ideal next post.