16GB is fine. My iPhone 6 has my entire music collection, photo collection and over 100 Apps on. Also all my documents with iCloud drive and OneDrive yet still has 1GB free and it's the 16GB version. Ever heard of the cloud?
Apple also use a technology called 'App Thining' to further reduce required space.
Some corporates also like to buy staff iPhones, if its just for email and a few apps on the go , 16GB makes sense too.