What is the purpose of using 8.33 kHz instead of 25 kHz frequency spacing?
The purpose is to accomondate more dedicated frequencies within the airband VHF range (117.975 to 137 MHz).

Increasing number of stations made this necessary, if the smaller division was not implemented, it was estimated that only 70% of future requirements for frequencies in Europe can be met.

833radio.com has further information and an nice table describing the channel spacing:


For simplicity, the actual 8.33 channels are displayed as rounded to .005 values.
For example to use the frequency 118.0083MHz, you dial 118.010