as the smoothness of something increases , the friction of that something will decrease & grease is used to make an object 's surface more smooth ->  grease is used to decrease the friction among objects  friction occurs when two object 's surfaces move against each other & wheels / gears usually move against other surfaces & a wheel is a kind of object & a gear is a kind of object ->  when wheels and gears move against other surfaces, frictions will occur  grease is used to decrease the friction among objects & when wheels and gears move against other surfaces, frictions will occur & a wheel is a kind of object & a gear is a kind of object -> grease is used to decrease the friction on the wheels and gears moving on other surfaces