The file abstraction is really a “stream of bytes” abstraction, which turns out to be useful for many things, not just file systems.
One example is the UNIX pipe, which is a simple form of inter-process communication. Processes can be set up so that output from one process is taken as input into another process. For the first process, the pipe behaves like a file open for writing, so it can use C library functions like
fprintf. For the second process, the pipe behaves like a file open for reading, so it uses
Network communication also uses the stream of bytes abstraction. A UNIX socket is a data structure that represents a communication channel between processes on different computers (usually). Again, processes can read data from and write data to a socket using “file” handling functions.
Reusing the file abstraction makes life easier for programmers, since they only have to learn one API (application program interface). It also makes programs more versatile, since a program intended to work with files can also work with data coming from pipes and other sources.