These Schottky diodes have a very low forward voltage drop compared to most. In the guitar pedals, these are very frequently used as reverse polarity protection diodes. So the (typically 9-9.6V) power from your power jack is routed through this diode first, then powers the 9V bus in your circuit (typical voltage drop in a 9V pedal circuit is usually <.45V). But if someone plugs in am AC/AC power supply to your pedal, it will have protection up to the reverse voltage rating of the diode, in this case, 20V. Not bad.
All of the latest GuitarPCB circuits use a 1n5817 diode for reverse polarity protection.
|1n5817 Schottky Diode
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(actual brand may vary)