It's for breaking in the cam and lifters. Otherwise, it's possible that you won't get sufficent oil flow and the lifters don't turn properly, which causes a wear spot on the lifter that flattens the cam lobes. Never let an engine with a new cam idle below 2,000 rpm. I've been told that for the first 200 or so miles not to let it idle for more than a minute or so. I did ruin a cam on a 250 once by not doing this break-in step.
