More information on Instagram having access to my person photos on my phones camera roll.
It's not Instagram as a whole which needs access to your camera roll but the Instagram app. Without the access right, the app will not be able to access the camera roll if you want to publish a photo taken outside of the Instagram app.

But even with this access right granted, you still are in control of the photos you share with your Instagram friends. They will not be able to access photos in your camera roll.