The channel 121.5 MHz is available only for genuine emergency purposes. To ensure that it is available, it is given extra spacing protection for 2 reasons:

1. To prevent the use of 121.5 MHz from interfering with other channels, and/or
2. To ensure that 121.5 MHz is always clear and that adjacent channels won't "bleed over" and make it unusable. This was more common in the past with older radio designs but although modern radios bleed much less, 121.5 MHz is still afforded extra band protection.

121.5 MHz is protected by not allowing use of 121.475 MHz & 121.525 MHz. The nearest available frequencies are 121.450 MHz and 121.550 MHz.

ICAO Annex 10 Vol V:

'In order to provide a guard band for the protection of the aeronautical emergency frequency, the nearest assignable frequencies on either side of 121.500 MHz are 121.450 MHz and 121.550 MHz'.