The power and majesty of the trinity has been a part of mankind for years, before the Internet. From religion (the Christian nature of the supreme being - The Father, The Son, and The Holy Ghost), to properly robust systems, using triple redundancy (as in Robert Heinlein's science fiction - "I tell you three times that this is so"), the number three has always had almost mystical significance.
And with your web site, there is a trinity too. You are using, and may be paying (directly or indirectly) for 3 different services.
If you have a domain purchased thru GoDaddy, and hosted on their services (maybe a blog published using FTP), you may have a package deal, with all 3 services on one yearly bill. Or you could purchase (register) your domain from any of dozens of registrars, have DNS provided by another company, and host your web site on a third, like Blogger. That's your choice, based upon your needs.
All Internet service companies don't provide all 3 services. GoDaddy does. Blogger / Google provides only content hosting, if you publish your blog to a Google Custom Domain.
You can register domain names from any of a number of different registrars, and you can use .com, .org, .net or any other valid addresses. Remember: you only need to get the domain name; you don't have to pay extra for hosting service.
When you purchase your domain, you want to know beforehand that nobody is already using it. And you want to ensure that, in the future, nobody uses it. You pay for the uniqueness of your domain name.
When you pay for DNS (directory listing), you want to know that your domain can always be located. Redundant DNS servers, geographically separated, is important for high visibility web sites. You pay for the address listing of your domain name.
Now this little detail may not be important to you.
Chuck, this doesn't matter to me. I pay my ISP for service, and they tell me to configure "ns1.myisp.com" and "ns2.myisp.com" (or their IP addresses) as my DNS servers. Why do I care about whatever DNS server Google uses?
Well, I'll ask you to think about this. You use your ISPs DNS servers so you can access websites. You pay for DNS hosting so your readers can access your website. Both your ISPs DNS servers, and your readers ISPs DNS servers, have to ask your DNS host for the address of your website.
You write your blog for your readers. Your readers DNS servers have to find out the address of your web site. How many repeat readers do you expect to have, if they try to access your web site and see
404 Server Not Availableor a similar error?
When you pay for Content Hosting, you want to know that your web site itself will always be online. Whereas a DNS retrieval is a small (but significant) amount of traffic, and of server space, your blog (web site), as it grows, will use increasingly larger amounts of server space, and generate increasingly larger amounts of traffic. Your content host needs a large and reliable connection to the Internet, as well as reliable server hardware. You pay for the hosting of the web site itself.
If you have a blog hosted on Blog*Spot, you pay nothing at all. If you have a blog hosted on a Google Custom Domain, you pay nothing for the Content Hosting, just for registration and DNS.
The latter 2 services are generally billed yearly, and at a fixed rate. Content Hosting is generally billed by the month, and will be tiered based upon the amount of storage required (size of blog), amount of bandwidth generated (number of readers * size), and various server services required.