The purpose of auto-trim is threefold:

 

A. To prevent snatching of the controls on disengagement of the autopilot

 

B. To re-position the horizontal stabilizer so that the control surfaces can remain in the neutral position thus giving you full movement on the controls.

 

C. To remove the standing load from the control surfaces.

Failure of the auto-trim circuit will prevent connection of the autopilot. (in simple terms - If the aircraft is not properly trimmed the autopilot cannot be engaged.)