This is a still from the film Dunkirk. I realise this is not a historical document, but I have also found a similar feature in photos of real Spitfires. What is the purpose of this?
According to flight manuals, it's the direct vision panel. The panel would have been punched out to make an opening and maintain a view on the outside in case the windscreen had been obscured for some reason.
Seen on either side, depending on the model
Some Spitfire models had it on the left hand side and this reproduction of the manual for model II actually mentions it on the port side:
Spitfire I: On the left side, source
It seems the side changed later, the official manual locates the panel on the right side for models V (5): VA, VB and VC.
The manuals are only vague about the reason the windscreen can be obscured, still a reason often cited the rapid altitude gain allowed by the aircraft creating condensation in the cockpit:
Note the punch-out panel on the canopy cover, an early solution to the new problem of canopy misting caused by the rapid altitude changes possible in the Spitfire.
Source: Flight Journal (pg 28)
Fogged windscreen is a plausible, as are a few other, cracked windscreen, oil spilled... we'll never know, online comments suggest it was never used.
Mins is right, but it's a more general thing: Glider planes have them too, or at least those built in the 80s that I flew in the 90s.
One of their effects is that they whistle in the (self-made) wind. Basically any deceleration is immediately noticeable in the note of the whistle going down (helps avoid stalling)... Relevant for a glider, but probably inaudible over a Spitfire's noise!
The ORIGINAL Spitfire I Pilot's Notes (as opposed to the concocted and suspect "1940 Spitfire Manual", whatever THAT means, quoted above) clearly state that the panel is for "Emergency use" and on the PORT side. "Port" is bolded in the text. The real para number is 45 in the Spitfire I and 37 in the Spitfire II Notes.
The panel allows airflow which may (or may not) clear the fogging of the front panel/hood and also could provide some perspective on landing to maintain the "triangle" of the runway edge and nose, should the hood be jammed. It also releases pressure aiding in getting the hood open in an emergency, necessary for vision or for bailing out...these early hood not having the Martin Baker quick release system.
The knock out panel was always on port (the photo above actually shows this but because of the thickness of both layers of glass, only the studs can be seen. If it were in fact on the starboard side, the outline of the panel could be seen.
The knockout was dispensed with from Mk V onwards. Some later manufacture Spitfire I's also lacked it, I believe, after the MB gear was introduced but have not been able to definitively confirm that.
The knock out panel was most likely meant to be broken with the pilot's elbow to equalize pressures, in order to allow the canopy to slide back for bailing out, as the suction of the air due to its shape was preventing it moving at high speeds. If the canopy could be slid back in flight in the first place, there would be no need for a knock out panel to clear condensation or oil or to look outside.