Well, you don't! But then no one will ever see your website, which would be a shame.
The physical website that visitors see when they click on your domain name is made up of a bunch of files and code. These files and code are your physical website. They need to live somewhere, otherwise they are just a bunch of useless files sitting in a folder on your desktop, only accessible to you, or the owner of said desktop.
To be seen by the public, the website files need to be stored somewhere central — on a web server. This is what your web hosting service provides.
Okay, you have your website files, stored in a central place, but people still can't view them because they need to a way to connect to those files. This is why you have a domain name. To keep it simple, a domain name is like an online address that connects the user's browser with your website files. They type in www.etc., or click on a link, and are immediately presented with the files and codes that belong to your website. Only, somehow magically all the gobbledegook contained within the files has been turned into beautiful (and useful!) text and images. This is what you know as your 'website'.
Note: Domain names are purchased for periods of time (a year, or a few years), and they need renewing. If they expire, and you don't renew them, the domains will become available for sale again, for anyone else to buy.
As well as storing and serving your files, your hosting service will usually include other options such as SSL certificates, domain specific email services and backups in addition to all the techie stuff that goes on in the background to keep websites up and running.
If you pay for hosting direct, and you don't have anyone managing your service, then you will need to do your own software updates to prevent your website files from becoming vulnerable to online hacking. This is especially important for WordPress website plugins which, depending on the size and complexity of your website, can require updating several times a month or more.
So, in a nutshell, to have a web presence, you need to pay for THREE things:
1. A domain name (which will need renewing every year)
2. A physical website to be built (unless you are building it yourself)
3. A hosting service (or a managed hosting service if you go through your website designer)