What's the purpose of my followers “focus”?

Question

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?

Answer 1

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.

Answer 2

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.