Most quality paving companies and all public roads use two coats of asphalt. A base coat and a top coat. The top coat or "wear" has smaller aggregate and more asphalt binders which makes it smoother and tougher and also helps keep water from penetrating through. However some paving companies will sometimes use a one coat system on driveways and such and simply roll out a base coat only. The base coat consist of larger aggregate and less binding agent and has a lot more surface gaps and crevices which will allow water to eventually pool up and penetrate. If your driveway is done this way I would recommend sealing it every three or four years to help keep water from penetrating. With a proper two coat system sealing is really strictly for aesthetics. Sealing any cracks is a must for longevity.
Dan
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