It is illegal and it also harms both the original website and the one that uses the copied content. Google sees that duplicate content and lowers the ranking of both sites for not having "unique" content. This is also a problem if you use an industry template website and use the template content instead of having new content written. There may be multiple websites in your industry with that same content. Your competitors who paid a writer to produce unique copy for their website will generally be more visible in searches than you when that is the case.

You can use Copyscape.com to check and see if your copy is located elsewhere on the web. 

BTW There are writers who copy portions of their content from other sites when writing for clients without realizing the issue. Request that your copy pass Copyscape as being at least 80% unique.