In reply to Marcus Tibesar.

I'll provide a simple example. Let's say you wrote a book called "Awesomesauce WP Code Recipes." Let's also say you have an existing WordPress site. Now, you could create a landing page on your website for that book. However, you really want that awesomesaucewpcoderecipes.tld domain for your book. Instead of launching an entirely new site, you can manage it from your existing WordPress installation.