What is the point of the planning time fence? Always greater than lead time?

I know what the planning time fence is, but I am just wondering a little more in depth why it is so important and what its purpose is. Is there a standard on what the PTF should be? I have heard it should always be at least equal to lead time. Say I am working with only buy parts in the system. The plant is a secondary plant, meaning it is not the home plant where the parts are bought. Will the PTF even matter? Say I set the PTF to lead time + 90 days...does that mean it is a buy part that has very long lead times and has issues with the supplier meeting the necessary requirements? What does a large PTF mean?

Thank you

-Mike