Since the disk is constantly rotating when the computer is on it would soon wear out if the read/write head were in contact with it.
Hard disks (and other secondary memory devices) are used for long-term storage of large blocks of information, such as programs and data sets. Usually disk memory is organized into files.
A file is a collection of information that has been given a name and is stored in secondary memory. The information can be a program or can be data.
Information in a file is represented the same as with any digital information — it consists of bits, usually grouped into eight bit bytes. Files are frequently large; their size is measured in kilobytes or megabytes.
If you have never worked with files on a computer before study the documentation that came with your operating system, or look at a book such as Windows for Dummies (or whatever is appropriate for your computer.)
One of the jobs of a computer's operating system is to keep track of file names and where they are on its hard disk. For example, in DOS the user can ask to run the program DOOM like this:
C:\> DOOM.EXE
The "C:\>" is a prompt; the user typed in "DOOM.EXE". The operating system now has to find the file called DOOM.EXE somewhere on its hard disk. The program will be copied into main storage and will start running. As the program runs it asks for information stored as additional files on the hard disk, which the operating system has to find and copy into main memory.
Where does a program typically put its results for long-term storage?