Part of the Unix philosophy is that every program should be a filter, one that can take input from stdin and output to stdout, so that it can be used in conjunc

Part of the Unix philosophy is that every program should be a filter, one that can take input from stdin and output to stdout, so that it can be used in conjunction with other programs (which may not have been written yet). If you wanted to add an optional feature to your program/script and didn't know what parts of it could be changed without breaking places where it was used, adding it as a non-default option allows you to do this seamlessly.