If you don’t have SOME domain name, then people can only visit your site with an IP address.
Additionally, you pretty much have to have a domain name if you want HTTPS encryption - if you don’t have an HTTPS certificate, people’s browsers will show lots of scary warning indicators on your page.
But if you’re asking about buying your own domain name (firstname-lastname.com) vs. using a subdomain from your hosting provider (myblog.wordpress.com) then it comes down to preference. Having your own domain will make you look more professional and get you more clicks on average.