I know it's probably a stupid question, but it always gets asked at the range, and the best I can figure is that it's for compressing the firing pin spring, which I've never actually tried.

It is for risk-free compression of the firing pin spring during a strip down of the bolt assembly.

Exactly as Will-Rogers said.  To disassemble the bolt you have to put the firing pin against something while you push down on the bolt sleeve compressing the main spring and allowing the cocking sleeve to turn.

www.surplusrifle.com/shooting/mauserboltdisassembly/

Instead of putting pressure on the tip of the firing pin (such as pushing it against a block of wood) which can break, the hole puts all the pressure on the main body of the firing pin which is much stronger.

LL