Not sure if this is well-known, might be but definitely some people don’t know it.
I have an iPhone with 16 Gb so I’m always dealing with low storage. A trick I do to get back some storage is to try to ‘rent’ a movie with a big file size (something like LOTR: two towers).
Just go to iTunes and search for the movie (with a bigger file size than what you have left on storage), click rent and soon you’ll get the message saying you don’t have enough storage and offers you to go to settings. Click that and magically you will get some storage back (probably it erases some useless data from different apps to try to download the movie).
Repeat the process 4-5 times and before you know it you recovered 1 to almost 2 Gb. Try it!