When you upgrade the miner to spelunker, he gets a fancy headlamp. As far as I can tell, when you use it, your mana drains but nothing else seems to happen. What exactly does the headlamp do?
The headlight itself lets you see better in dungeon area, but it is not mandatory really, you can even simply increase your screen brightness for a similar effect.
But the Spelunker upgrade also grants your miner the passive ability of locating rooms with treasures, this ability doesn't cost mp and has nothing to do with headlight.
So even though headlight isn't really useful the Spelunker upgrade is probably worth buying.