The emphasis here is on practical usage/benefit (productivity, efficiency, simplicity, etc.).
By surprising, I mean what has shown you the greatest returns, though it was not an emphasis in a curriculum or hiring.
For example, something you learned when you were 12 you found not many people understand or use, but has served great productivity. Or it could be something that everyone knows, but doesn't use because of a trend, or some other reason.
It could be a language or a tool, just something technical that you never realized would be so useful.