The subject of this set of tutorials is Linux system calls. This, after all, is the main interface between an application program and the services provided for it by the kernel.
One of the basic philosophies behind Linux (in fact, behind all flavors of UNIX) is that the system will try to make its input and output to all kinds of devices, like disks, CD-ROMs, terminals, printers, etc., look as far as possible like input and output to ordinary files.
One of the most obvious reflections of this is that the system calls which are available for use on ordinary files will keep cropping up over and over as we look at driving different device types.