Eina tmpstr is intended for being able to conveniently pass strings back to a calling parent without having to use single static buffers (which don't work with multiple threads or when returning multiple times as parameters to a single function.
The traditional way to "return" a string in C is either to provide a buffer as a parameter to return it in, return a pointer to a single static buffer, which has issues, or return a duplicated string. All cases are inconvenient and return special handling. This is intended to make this easier. Now you can do something like this:
Notice that you can interchange standard C strings (static ones or even generated buffers) with tmpstrings. The Eina_Tmpstr type is merely a type marker letting you know that the function will clean up those strings after use, and it is totally interchangeable with const char.