Imagine this scenario: a Virtual Machine running on a VMware ESX host. In virtual hardware I assigned only 1 CPU (1 core). It is running Windows Server 2008 R2.
Now let's say there is an anti-virus client application on this VM which sometimes takes up 100% of my CPU, causing the entire VM to crash to a halt (it takes around 10 minutes just for the Windows logon page to show in the VM console, and RDP is impossible).
So I go to the process explorer and see that the process "System" is taking up 100% of the CPU. Well, that doesn't make sense, so I go deeper and I find that there is a particular thread which is the cause of the problem, and the source of this thread is the anti-virus application. It is a known bug from this software with no available fix.
Now if it were a lone process, I could limit the CPU utilization of that process and...