Declaring free functions as static gives them internal linkage, which allows the compiler more aggressive optimizations, as it is now guaranteed that nobody outside the TU can see that function. For example, the function might disappear entirely from the assembly and get inlined everywhere, as there is no need to provide a linkable version.
Note of course that this also changes the semantics slightly, since you are allowed to have different static functions of the same name in different TUs, while having multiple definitions of non-static functions is an error.