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.)