Why use the Hall effect?

The reasons for using a particular technology or sensor vary
according to the application. Cost, performance and availability
are always considerations. The features and benefits of a given
technology are factors that should be weighed along with
the specific requirements of the application in making this decision.
General features of Hall effect based sensing devices are:

• True solid state
• Long life (30 billion operations in a continuing keyboard module
test program)
• High speed operation - over 100 kHz possible
• Operates with stationary input (zero speed)
• No moving parts
• Logic compatible input and output
• Broad temperature range (-40 to +150°C)
• Highly repeatable operation

Source ( pdf )
http://www.honeywell-sensor.com.cn/prodinfo/magnetic_position/
technical/chapter1.pdf