I sit on the East Surrey Transport Committee and this wretched fence comes up at almost every meeting.
As you surmise, its purpose is to discourage suicides on a stretch of line that has been a suicide hotspot and to some extent it seems to have succeeded or at least moved the problem elsewhere.
However, the Health & Safety people have decreed that, because space on platform 2 is limited, the only gates in the fence must be at the far ends of the platforms - to the great annoyance of passengers who find themselves with a long trek to the end of the platform. There have been cases of people missing their trains because they didn't factor in a 200-metre dash to the end of the platform. It can also cause chaos on Sunday, with passengers milling about looking for an exit.
The Elf & Safety lot (actually, Network Rail's safety people I presume) say having gates only at the ends is necessary to allow a complete trainload of passengers to disperse. As platform 2 is the down fast at Purley and most trains that stop at Purley use the slow lines except a handful in the early mornings and during weekend engineering works, you might think this would not be an issue but, say Network Rail, 'What if you had to evacuate a complete train-load on platform 2 in an emergency? You would need to make passengers disperse along the length of the platform rather than crowd around a central gate.'
True enough, we say on the Committee, but as Purley is a permanently staffed station, if such an event were to occur couldn't a member of the staff lock the middle gate? This has been rejected, as has another suggestion, that given the low numbers using the trains when Platform 2 is in use, a direct route from could be provided by unlocking the waiting room doors that front onto that platform, providing a short-cut from platform 3 to 2. (This might not work on Sundays when the trains are quite busy.)
Members of the committee continue to press the issue, though don't hold your breath...