Note that it is not exactly same as copyright infringement.
Copyright infringement is violation of using copyright holder's content without his consent.
But Plagiarism is concerned with reputation of author which can be affected if his content already available in somewhere.
Say for example, you are publishing some content. And, the same content is already available in another website. So, the reader of your site who already visited the other site will be thinking that you copied the content from other site which ultimately reduce your reputation.
Normally, it will happen if you are outsourcing your writing work. Assume that you are asking some employees/contractors to write articles. If they are giving articles just by copying it from some sites, your reputation will be spoiled.
So it is very important to prevent Plagiarism to save your reputation or good name.
We can detect the duplicate work using service provided by www.copyscape.com. Just entering url of any webpage in this website will return list of already existing webpages having same/similar content.