Why does Nascar seperate the field before the start?

May be...

Just like the first two said, they do it to get their tach readings. If they put the whole field together they cars at the back will end up have a different reading, the whole accordion effect thing.

So they can set their tachs for pit road speed,most "parade/pace" laps are run at pit road speed.

.of the race into two groups and then bring them together before the green flag?

To let them match their RPM's with the pit road speedtelnet ipaq