@bu5hm4n A practical example is because of this https://godbolt.org/z/hGcc4m .
(on the left, there isn't a declaration, on the right, there is it).
C compiler only see names declared above from the current name, so, for example,
in eina_accessor.c, we have

Eina_Bool
eina_accessor_init(void)
{
   return eina_magic_string_set(EINA_MAGIC_ACCESSOR, EINA_MAGIC_ACCESSOR_STR);
}

Eina_Bool
eina_accessor_shutdown(void)
{
   return EINA_TRUE;
}

so, inside eina_accessor_init, it could be possible to do things like
(highly dependable of c and std version)

Eina_Bool
eina_accessor_init(void)
{
   eina_accessor_shutdown(1,2,3,4,5);
   return eina_magic_string_set(EINA_MAGIC_ACCESSOR, EINA_MAGIC_ACCESSOR_STR);
}

But this is positional, with definition in the beginning of a file, it's possible not have the declaration,
for safety, it's good to have declarations.