Digital hygrometers usually also have other uses, such as measuring temperature and other environmental factors in addition to the humidity level. There are some pros to using these kinds of tools, though. For instance, they are very, very accurate, and they are often more affordable than quality analog versions.

How accurate are cheap digital hygrometers?

Cheap mechanical hygrometers and cheap electronic hygrometers are supposed be accurate within +/-7%.

How do I know if my hygrometer is accurate?

Dampen a towel (not dripping wet, but good and damp), then wrap the hygrometer in the towel for 30 to 45 minutes. Then unwrap it and read the humidity (quickly). If your hygrometer is perfectly calibrated (few are) it will be reading exactly 100% humidity. Most likely, it will be reading somewhere between 80 and 90%.