Text files can be present in different encodings. In recent years, the situation has greatly improved, but there are still compatibility problems when working with different systems.
There's a difference between raw data in a file and a string object in Python. The string object has been transformed from whatever encoding the file contains into a native string. Once it is in this format, it may need to be stored in different encodings. By default, Python works with the defined by the OS, which in modern operating systems is UTF-8. This is a highly compatible encoding, but you may need to save files in a different one.
