There are still WW2 pillboxes all around where I live in he South of England. What was the point of these? Was it just 'theatre security'? It's hard to imagine one tiny little brick structure in a valley stoping an advancing army.
The idea behind a pillbox or any static defense, is that it isn't necessarily made to be relied upon to solely defend against and stop an enemy advance all by itself.
Typically, they were constructed or deployed in conjunction with other nearby defensive structures, features (terrain, road layout), or force deployments in order to help strengthen an area's overall defense. The other idea behind defensive fortifications like pillboxes is that they weren't necessarily constructed with the intention of only being useful if they could stop an advancing force. They were constructed with the understanding that even if they failed to halt an advance (which is a highly probable outcome given enough time and resources on the part of the advancing force), they could either slow it down or force the enemy to shift their forces in such a way they hadn't anticipated, giving the defenders time to either reinforce the established position or to fall back.
In regards to their use in England when the fear of a German invasion in 1940 was high, you could argue that it partially was "security theater" doubling as defense. Though I would argue that after Churchill's We Shall Fight on the Beaches speech, the British were well prepared (from a morale standpoint, materially they were in a slight bind for a few months after the evacuation of Dunkirk) to make an invasion of the UK as costly and hard fought as they could for German forces if an invasion was carried out. Pillboxes and other defensive structures would have been useful in slowing down and bleeding out a German advance even if it resulted in the British Isles eventually succumbing, even though the likelihood of that happening let alone the invasion itself being carried out were highly unlikely.
a single pillbox was unlikely to be able to hold off anything for long, the genius is using a pillbox in conjunction with other structures.
British defences were divided into things call stop lines (such as the GHQ line) which stretched across the country, these were there to slow a German advance, as you got closer to London they became more potent. They would probably have been most effictive at stopping tanks, not necessarily destroying tanks just inhibiting their movement so they could not run riot as they did in France in 1940 and Russia in 1941. They were basically blitzkrieg barriers.
https://www.ibiblio.org/hyperwar/UN/UK/UK-Defence-UK/img/DefenseOfUK-14.jpg
---> http://www.pillbox-study-group.org.uk/uk-defence-locations/ <---
This is in opposite to the Germans approach which seems to have been more of an "all the eggs in one basket" kind of deal, their heaviest defences were on the beaches and the interior had barely any static defence at all. I live in the Channel Islands, one of the most heavily defended sections of the Atlantic Wall and there are almost no defences on the interior of the islands, its all on the beaches. In Normandy the situation is quite similar as far as concrete goes.
