There are many ways to detect if an article is original or just copied from somewhere else. With the ease of finding unique articles on the web it can be tempting to reuse one of these under your own name. This is a bad idea especially with the abundance of plagiarism checkers. Not only are there many options, but also many are free with paid subscriptions available for heavy users.
High school teachers and college professors to check if a paper is original or copied by a student often use popular plagiarism checkers. This has helped thwart of cheaters in many schools; forcing students to write their own papers.
Another major use is by search engines, article writes, bloggers, etc. Anyone who published content on the web should be using a plagiarism checker online to ensure their content is original and not stolen from outside sources. This is because duplicate content will not rank online as the search engines want to see original content and not rehashed content.