Image files, audio files, executable files from other programming languages, many kinds of data files, ... the list is endless.
So a stream object may be:
There are many types of streams. For example, a stream might be an "input, character-oriented, processing stream". Another stream might be "output, ordinary, byte-oriented stream". Not only are there many types of streams, there are many ways to connect them together. The IO aspects of a program may take as much work as all the rest of the program!
This situation is common to all production (industrial strength) programming languages. IO is a big topic because of the wide variety of IO devices and the wide variety of data formats. A production language must be able to read data from sources written by any language running on any type of computer. For example, a program might need to read data from magnetic tapes written 30 years ago by a COBOL program running on an IBM mainframe. (You would likely use byte-oriented streams for this).
The good news is that Java uses the same data formats for all computers. If a Java program running on a Macintosh writes a text file, that file can easily be read by a Java program running on a PC or on any other computer.
This is not true of most other languages. A "C" program running on one type of computer is unlikely to create a file that can easily be read by a "C" program running on another type of computer. Worse than that, two different "C" compilers for the same computer are free to use different data formats!