SELinux is an absolute must in todays world, simply put it prevents applications and namespace from doing thing outide of their area of interet, for example a web server should never be allowed to execute files from /tmp so it prevents that, another great example is when you have any form of multi-tennancy - for example you might have several docker or LXC containerised apps running on a host SELinux will / can ensure that even if of those apps is compromised and someone popped a shell in it, they couldn't go beyond anything running in that namespace, i.e. to the host itself or other apps running in containers on the same host.

It prevents a lot of sitations where an app might behave badly, for example perhps you're running a PHP app and a 0-day PHP flaw is discovered, there's a good chance that SELinux restrictions could prevent that exploit from being useful to the attacker by not allowing the app to do unusual things.

We have a rule - if it's not running SELinux in enforicng then it's not going into production.

We use to run GRSec as well, however with modern kernels a lot of the GRSec code base has been merged in so it's often not required.

grsecurity is enhancing the security of the kernel itself, SELinux is for user space applications.