Unknown data taking up my iPhone's storage

Hey there,
I have 6.8 GB used space on my iPhone 6 (according to Storage menu in Settings). However, I counted all the apps together and they take up only 3.5 GBs. What's the reason for those additional 3.3? How can I get rid of them? I'd love not to be anxious about my storage as much as I am right now. I can't really put all my favorite songs and pictures on my phone as I almost run out of free space that way.

Thanks in advance