The title says it all. And just in case you didn’t know, @Wayne_Small did a very detailed description on what the holes are for here.
But if you read the question, you see this is not the answer
The developer switch’s use is currently undocumented so we really don’t know? My guess though is that pressing it causes an NMI (non-maskable interrupt) so it can create a breakpoint while a problem is occurring, and transfer control to debug software for the programmers to examine the memory and registers to troubleshoot errors in the code? Similar to the programmer’s switch on the early Macintosh computers which caused it to stop what it was doing and drop into a machine language disassembler. In the future they could program it to also do other things like a factory reset for example. Again it’s just a guess on my part though, and what I would have used it for.