What's the lunchbox for?

Picking up a Vault-Tec lunchbox will open it and spawn a random item, which will fall out of the lunchbox and be on the floor somewhere near by. You get to keep the lunchbox after opening it and it can be used in crafting.

From experience it's largely junk items that you can use in crafting. It's possible to save your game before opening the lunchbox and reload it to try for another item as the item is spawned at the point you open the lunchbox.

When you activate a Vault-Tec lunch box, an item may pop out for you to pick up. My experience indicates that this depends on your character's Luck.

This page at Fallout Wiki lists some possible items you might find. I don't think it's a complete list. That page makes it sound as if something always pops out. This may be the case, but I remember having nothing pop out before. Perhaps I just didn't notice it.

The item is usually some junk, like a spatula. You can still pick up the lunch box, whether you pick up the item or not.

The lunch box can be used to make bottlecap mines.