There is another reason: to make sure the program doesn't do anything funny with the file, like modifying it. I know it won't happen with "head" but for command

There is another reason: to make sure the program doesn't do anything funny with the file, like modifying it. I know it won't happen with "head" but for commands I am not familiar with, it is a way to be sure. And example of a command that does "something funny" is gunzip. With just a file as an argument, it will decompress the .gz file and erase the original instead of reading it and dumping its content to stdout. I usually prefer to do "< file command" though. "cat" adds an extra layer of indirection, forcing stream processing and hiding the original file, but that's usually unnecessary. If you really don't trust the program, it is not an adequate solution anyways.