This option enables automatic generation of order’s magic number. If you have multiple instances of the expert advisor running simultaneously (e.g.: you may have 3 charts opened - say EURUSD, NZDUSD, GBPUSD - having the expert advisor attached in each one of them), then you need to have a different magic number for each one of them so that each instance of JedeyeFX do not interfere with each other.
Therefore, this options saves you time and concern as it automatically generates a different magic number for each expert advisor instance. You can check the current magic number at any time by looking at the Information Box in the right upper corner of any chart where JedeyeFX is attached to.