There are plenty of reasons that if there are a built-in telephoto lens in your phone, why you still need an external 3x Telephoto Lens?
1. the built-in lens may just got 2X zoomed
2. this added on 3X lens will have better image
3. with the 3x attachment kit (say, a cover or a clip), you can have more different lenses for stunning pictures.
with an iPhone cover:
4. when you take pic of portrait, you will need one.
We have many different covers for iPhone for this lens and also universal clip for other smartphones. Don't miss to have a look.
Here is the link to the telephoto lens: