In order for the rest of the world to visit your website, your site has to live somewhere. Traditionally this is called a “web host.”
Hosting a website requires a lot of other pieces of software besides just WordPress: you need, at the very least, webserver software and database software.
In order to make your site respond quickly, you need other pieces of software for caching. Of course, you also need it to be secure, and that means all of these pieces need to be set up properly.