way back up the thread the statement was made that using SELinux for many processes on one machine was as secure as having the processes on separate machines separated by firewalls.
This is an example of capability that you could have to filter communication between apps on different machines that you do not get with SELinux securing things on one machine.
as for what this would be useful for.
if you have apps that expect things to be text files and throw arbitrary binary data at them you may find a flaw in them and be able to do things as the owner of that process. If you make sure that such bad data can not get to the app you eliminate an entire class of exploits.