What is the utility of having static functions in a file?
You can use these functions to provide shared implementation logic to other functions within the same file. Various helper functions specific to a file are good candidates to be declared file-static.
How are they different from having global functions in a file?
They are invisible to the linker, allowing other compilation units to define functions with the same signature. Using namespaces alleviates this problem to a large degree, but file-static functions predate namespaces, because they are a feature inherited from the C programming language.