Domain Names are "symbolic names" for IP-addresses (IPv4 or IPv6). When machines are addressed over the internet, the connection is established by using the IP address. As those numbers are hard to memorize, the domain names were invented quite some time ago.
When you own a domain, you get basically a namespace below one of the top-level domains. The domains are issued by authorized registrars.
So let's assume you aquired
It is then totally up to you, how you use this domain. You can use it as is (without any sub-domains) or you can add multiple hostnames (still without sub-domains) to it or you can go beyond that and create a sub-domain structure.
Example with multiple host but NO sub-domain:
Example with additional sub-domains
Is a sub-domain required: NO.
Is a sub-domain useful: sometimes, depends on your needs.