Why do you need a domain name?
Why do you need a street address? so people can find you or your store or send you mail.
Your domain name is your IP(internet protocol) address. It is as if you were living in an
apartment building. Your address would be unique to your apartment. Everything in your apartment is yours. The way you decorate it, the food and drinks you serve your guests and how you serve your guests is uniquely yours.
Unlike moving into an apartment and being stuck with the address you are given on the internet you can actually pick the address you want. I am not saying that someone else hasen't picked the address that you want. You just need to investigate the address you want and the variations possible.