why would a check valve be necessary?

All 1/2" steel piping with #2 fuel oil flowing

There is an
actuated shutoff valve then about a 20' run of pipe then a 10' drop and
another 8' run to an oil gun with tip out in the furnace.

There is
a negative pressure coming from the furnace, and there is a small
additional negative pressure created during a 30 second purge with the
oil off and air still on... probably equals about 3 psi total...

So why would there need to be a check valve installed immediately before the oil gun?

Whouldnt
the pressure after the valve be 0 and the pressure from the atmosphere
be like 15-3 = 12psi + friction and such... so the oil would all stay in
the line except for maybe a little that would come out that was in
around that last 8' run?

I think a check valve would do little to
almost nothing here... a couple people are insisting that there HAS to
be a check valve there... am I missing something? 

It is most likely to prevent the atomization air (compressed air)
from backing up through the burner nozzle.   Fuel oil and compressed air
connections will be near the windbox, and these go to the oil gun which
uses the pressurized air to atomize the oil.

I am not familiar
with the specifics of how oil burner guns are configured, but it seems
possible that the air could backup through the oil line if the oil pump
was suddenly shut off or a valve closed.
The primary purpose of the check valve may be to introduce a forward
pressure drop (cracking pressure to open the spring) and therefore
prevent the oil lines from draining into the furnace when it is
shutdown.
For one thing it will keep air that replaces the oil that has run out
the end of the pipe from making its way back to wherever it can make its
way back to.

From the originalCheck Valve
.