Your phone stays dry and separated from your other items, and bonus, you can use it! Front facing camera only, but maps, messages and phone all usable through the clear front.

Your phone will not be scratched by your keys or smooshed with your sandwich.

Dry bags are great for keeping your stuff dry when things go wrong - capsize, falling in, rain, splashing, swamping etc. They are NOT for scuba-diving or any kind of prolonged immersion.

If you trap some air in the bag, it will also float.
I strongly recommend the dry bag is connected to you or your kayak / SUP in some way. It might float, but you don’t want it to float away!