Let me give you some analogies.
You walk into an office building and ask the receptionist:
Where is the office of Mr. X?
The receptionist can look at some list and say, "It is located on the 3rd floor at suite number 3".
You now can go to that floor and office and sit down with Mr. X at his office on a nice couch.
Now, we have several elements here.
1. The receptionist is the domain registrar. She knows what is the name of the websites, in this case, the name of Mr. X and has a location.
2. The suite number 3 on the third floor is web hosting, where you can have your office.
3. What you have in your office is the content. If you have some nice coaches, a desk, a store or a warehouse is up to you.
4. You can make them yourself, or buying something pre-made. That is the platform you use.
Godaddy is the registrar, that also provides web hosting if you want, and you can install WordPress there or not.
Wordpress is a platform to manage content, you can use it on wordpress.com or take the pre-made software and install it wherever you want.
Some hosting companies can install it for you as a bonus.
Wordpress can also buy a domain for you, you do not need to do it with GoDaddy.
Does GoDaddy not have everything I would need to build and maintain a website?
You could have the receptionist, and the office and the pre-installed Wordpress. Is everything you will need? You probably need a web designer, or a store manager, or a content manager, or a writer... all depends.