What is this hole for?

you shouldn't use oil in a lock, graphite spray is best

take it from a locksmith graphite.

Do not use graphite in locks used outdoors. When it gets wet, graphite has a tendency to turn into sludge. Manufacturers recommend silicone or Teflon lubricants. WD40 will work short term, but is not intended to be a long term lubricate.

Player #8297262, A newer synthetic lube is better. To help the key turn, place the lube in the keyhole, not the drain hole.

who knew????

see unless info of youtube does have a use