web hosting – From a technical standpoint, why do we have to pay for any domain name?

In terms of the ownership of any domain name, why is it that a person or a company who owns a website and the technology to make the said website work has to pay “extra” money to “rent” a domain name?

Do these (domain registrar) companies offer you some kind of protection against DDOS attacks or do they own some kind of more powerful servers to relay connections towards your main server where you host a site?

And the last thing, do these companies grow/increase prices for more popular domain names and why?

Thanks in advance!