Dennis got an iPhone 6s Plus a few months ago and he can't seem stop the storage from filling up his phone. Leo says that the iPhone caches a lot of data. If he plugs it into his computer, he can take a look at it in iTunes. It will show him a graphical representation of the space used in "other." The only real way to clean it up is to backup the iPhone, erase it, and then restore it. That will get rid of all the cached stuff. He can go into Settings > General > Storage and Usage, and clear everything out piece by piece. The only way he can truly get rid of it all is to backup the phone, restore it, and start over.