SELinux (as well as AppArmor) are used to restrict the actions of users and processes on the system. A classic example is that the httpd process should not have access to files outside of the usual
This means that SELinux is the last defense on your server that prevents an attacker from doing worse things than what is available to the exploited process already. If you have properly configured services and updated packages, then SELinux has no advantages outside of any potential 0day flaws.
There are tons of servers that don't use either of these technologies. SELinux in particular has a bad rep for being difficult to configure so you'll see recommendations to disable it as soon as you install a new system (I disagree with this). Also, loads of VPSes that run limited virtual machine images do not come with SELinux support so there are tons of web servers out there not using it.