Linked from
The 9 pages that link to Gear ratio, each with the reason it gives.
GearsRelated: It quantifies how a gear pair changes speed and torque.
Rack and pinionRelated: The pinion’s size determines the rack’s travel per revolution.
Revolutions per minuteRelated: Gear ratios determine how input rpm changes into output rpm.
Flexible shaftRelated: Gearing often sets the speed and torque delivered to a flexible-shaft input.