With level 18, you unlock a new item slot in your followers inventory:

However, neither the game nor much of goole makes any mention of this. What's the purpose of this focus?

2 Answers

It is simply a follower-specific piece of equipment, to give them stats and bonuses, same as any other equippable item.

Templars have Relics, Scoundrels have Tokens, and Enchantresses, like you've discovered, have Focii.

Focuses are primarily highly symbolic personal effects. They serve to help channel unusual enchantments.¹

They're "Follower Specials" equivalent to the Scoundrel Tokens or Templar Relics—an otherwise useless item that offers passive bonuses. The Blizzard site has a list of usable focuses.