Asphalt is a porous material. When an asphalt drive is sealed, it keeps the salt and leaking oil on top of the surface.  Sealcoating also preserves your asphalt drive.  Asphalt is a mixture of stone, sand, and a binder that keeps it all together.  That binder (called asphalt cement) breaks down and evaporates from UV rays. As the asphalt turns from black to gray, it's the binder evaporating from the surface.   
 
Sealcoating your driveway slows that oxidation.  Sealing can make what you have last longer.  If what you currently have is 20 years old and is in rough shape, sealing will help improve the look of the drive but it will still be 20 years old and rough shape.  At some point in time, an asphalt surface will need to be redone.
 
So what can sealing do?  It can protect your drive from contaminants, slow oxidation of the asphalt, and improve the general appearance of the drive. What sealing can't do is take an existing drive and make it brand new.