While the top level domain is essential for a domain to function, it's also less exciting than the second level domain. Which is probably why it's sometimes referred to as the "parent" domain.
The exciting thing about domains is that you can choose almost any name you want as your second level domain. And sure, you can go with .com for your top level domain, but who doesn't want a cool "parent"? Options like .club, .store, even .pizza are just a few ways you can boost your top level domain game.