Hi! Popper.js dev here.

There are several advantages:

- The size is half of Tether (3.5 vs 7);

- Popper.js doesn't need any additional CSS in your page;

- It works even with elements not placed as direct children of body;

- It's much much faster (compare both on mobile, you'll see Tether lagging a lot!);

- It automatically detects the size of the tooltip arrow;

- Inside Popper.js you already have a way to create tooltip elements automatically, with Tether you need an additional library;

- Popper.js supports particular "flip" behaviors, you can decide that if there's not space on the left, the tooltip must move on the bottom (for example);

- The whole code is fully documented with JSDoc!

I hope this answers your question