It's cheap insurance against premature wear. There will be a short delay during startup for your rotating assembly to receive the oil, so the assembly lube acts as a "buffer" until the oil reaches the bearings. Engine oil isn't very resistant to surface rust either. I took my valve covers off for powder coat and my cam lobes developed slight surface rust within 1 day, even though I poured 10w30 on them! I used steel wool to remove the surface rust and applied assembly lube..no rust for over 1 week now!