HID devices can be used for all sorts of applications, like these:

• data acquisition devices
• test instruments
• data loggers
• devices to control hardware
• display devices,
• and of course keyboards, mice, and joysticks.

In fact, just about any kind of device that doesn't already fit into some other existing USB class, and in which a 64,000 byte per second data rate is acceptable, can usually be done better, faster, and more intelligently as a USB HID class device. And it will never require a special device driver to be installed, either.