The preload is there to allow the inner plunger of the lifter to run in the center of its travel. That way, there is never any loss of contact between the cam lobe and the lifter, and the overall length of the lifter and pushrod is able to compensate for pushrod and valve stem stretch and shrink due to heating and cooling without rattling or holding the valve open. The internal alignment of the lifter body and plunger is also best with the proper preload so any seepage of oil can be replenished by the oil pressure when the lifter is on the base circle of the cam with minimum load on the valve train. Since each lifter operates 60 times per second at 2K RPM, there's very little refill time available with the cam lobe unloaded.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!