because paint isnt smooth mate, its slightly rough, so when you polish it, you fill some of the gaps in, making it look all nice and glossy (to do with the way the light bounces off it or sumthin) and when you use a sealent, its like another layer ontop of the polish, to make it even more smooth, and i think its made up 'harder' so aswell as another 2 layers, it adds protection to the paint.
As far as Im aware anyway, I could of just said a complete load of b0llocks though...
And some, if not most sealants can be waxed over as well, to get an even deeper, longer lasting shine. I use car-lack 68 to polish which puts down an acrylic layer, then megs nxt wax over the top.