Security implications

The fact that processes can't communicate or interact without specifically enabling it leads to security as a pleasant side effect!

Processes can't affect memory of other processes
Limits memory-leak related crashes	
Your web browser can't access your email program and spam your friends

When restrictive file permissions are used, one user/program can't access another's data inappropriately
You can create files and directories readable only to you, or certain groups, with fairly fine-tailored control.
A malicious user can't directly access the database files, he must go through a properly authenticated connection