Hi Patrick
Think of a Domain as a mobile phone…
Sure you can always “borrow” a phone and make a call. Only no one can ever call you back…
If you get the cheapest mobile and rate, the whole world can call you!
How many services does eg Apple provide under apple.com? Or Microsoft.com?
With just one Domain, you can have millions of subdomains…
Like this:
mydomain.com = your domain
- www.mydomain.com
- mail.mydomain.com
- nextcloud.mydomain.com
- wiki.mydomain.com
- ha.mydomain.com (Home Assistant)
You can do as many as you want or need… 
So treat yourself to a real domain! (I also have my own domain, as you’ve seen…)
My 2 cents
Andy