why it is'nt good to restart the computer by using the reset button on the computer case?

When Windows is running, many files are opened and in use by the operating system and with the programs you have loaded at start. These files are marked as open in memory, and on the hard disk. When you shut down your computer normally, Windows saves any changes and properly exits all running programs and files, before shutting off the power.

When you press the reset button, or pull the plug, or there's a sudden power outage, all these open files are left open. This can result in a critical file, or the boot disk becoming corrupt, and can result in a computer that does not boot next time you try to use it.