You could think of the sites that share your server as your roommates; there's really not that much separating you from them. Sure, you can close the bedroom door, but they can still cause nightmares for you in the kitchen and the bathroom. In web hosting terms, all the sites share a single server's resources, so huge traffic spike on Site A may impact the neighboring sites' performances. It's even possible that another site could take down the shared server altogether, if it crashed hard enough.
CloudLinux helps protect websites in a shared website hosting environment. If one of the websites on the servers gets attacked or gets a sudden growth in popularity and a lot of unexpected traffic, it will start draining all of the server resources, affecting other websites. CloudLinux makes sure this doesn’t happen. It will either slow down the website to stop it from hogging all resources or it will temporarily shut it down.
Think of your domain name as a street address for your website. Without a domain name, you would have to tell customers to visit your website at an IP-based url such as 184.108.40.206/~yourwebsite instead of yourwebsite.com. The former is not very memorable or friendly to human eyes and will surely result in not that many visits to your website. While it is not mandatory that you have a website domain name, nor is it mandatory that a domain you own be attached to a website, for a successful online presence you truly will want to get your own domain name to ensure your website has the proper air of professionalism and appropriate branding.