Why zero backlash isn't automatically the best setting — and why a little free play is usually the correct design choice, not a defect.
Every pair of meshing gears has a small amount of clearance between their teeth, called backlash. It's invisible while the train is turning in one direction — but the instant it reverses, the driving gear has to rotate through that empty clearance before its teeth find the driven gear's teeth again on the opposite face. During that gap, nothing downstream moves at all, even though the motor is turning. It is possible to design that gap away almost entirely, using preload. That doesn't make it the right call for every gear train — eliminating backlash trades one problem for another.