Just running with the military idea (gun emplacement etc) the suggestion is whatever it is is on the far side of the rock and therefore hidden from our view - otherwise we would see it, or the remains of foundations etc that supported it.
I cannot see any evidence of a path leading more than a couple of feet from the seaward end of the bridge. Therefore to cross to the far side of the rock it would be necessary to pass through some kind of short tunnel, the entrance of which is hidden from our view. Something I've seen in other situations and therefore not necessarily unlikely. Given the scale from the height of the handrail any such tunnel would have to be at a lower level (and falling away) than the deck of the bridge, as head height of someone standing on the bridge would be close to the very top most point of the rock.
I've added a speculative 'hidden' tunnel, reached by steps, to the picture.
But I really don't think this is what is there for two reasons:
1) It is a small piece of rock. Tunneling through it would be hard work, possibly requiring explosives. Far easier to use explosives to simply blow the whole of the top of the rock away, and if intended for military use then explosives may well be the first thought of the engineers!
2) As posted before, the rock is probably sedimentary and made up of layers with sloping bedding planes. This creates a risk of slips. Imagine the rock as a pile of jenga blocks with the pile being slightly tilted. Nothing stops a block sliding off the one below, other than the friction between them. On our rock the two green arrows indicate a bedding plane I'd be concerned about. If that plane extends throughout the whole cross-section of the rock, then the only thing holding the top of the rock in place is the friction between the two layers. Tunelling through the rock at that level would reduce the frictional force and increase the risk of a slip. There are (modern) engineering solutions to reduce the risk (rock bolts), but often the simpler and cheaper solution is still just to remove the risky material (i.e. blow it up).
So for those reasons I don't think the bridge provides access to the rock to provide access to something else. There are other more practical ways of getting to the other side of the rock if that is where you wanted to go.
In terms of an observation point, why go onto the rock? Simply build a low structure (say 10 feet) where the walls are on the right hand side of the photo and you would get as good a view as you would sitting on the rock. Searchlight? Why would you need one before aircraft were a problem in WW1?
Mike's comment on secrecy is also relevant - if this is a military installation in active use, then would a photographer be allowed to photograph it? The condition of the walls and bridge suggest something fairly new, not old and disused.
However, my previous question about why it isn't a well known location today might be answered if this area has since become part of a military establishment - any tourists would have been banished if the area is now behind barbed wire and 'MOD keep out' signs. There are plenty of locations (like Orfordness) where that has happened since Edwardian times.
