An organist has a third hand, represented by both feet.
This third hand, using the proper footwear, is capable of playing 4 and 5 note spread chords if necessary, all by itself [See blog, Balance in Organ Playing, Part I].
Intervals of thirds and fourths can be sounded by the same foot by wearing a shoe that's built up at the heel [See blog, Shoes, Part I, II] and simply stepping across, turning the ankle outward, and missing the middle note in the arch of the sole.
By drawing the appropriate pedal stops, these spread chords can then be made to supply any pitch and fit anywhere in the harmony, even above the melody, if desired (a raised shoe heel makes pedaling more deft in general because it permits more rapid execution of scalar passages, heel to toe, where there's less foot movement from the ankle down).
All of this is extremely important to good practice [See blog, The Ten Commandments Of Organ Practice].
No other musical instrument in the world works like this.