the reason for storing wine laying on its side with neck up slightly is so the corks stay moist and not drying out which would cause them to change slightly in size and also start to crumble. So by keeping the corks damp this allows the wine to remain sealed and the bottle in a proper cellaring are rotated on a regular bases to insure all the cork remains moist. The point at which the bottle is laying is also noted before serving so any sediment stay in the bottom corner and not agitated to muddy the wine
The cork exposed to the air allows ever so timy amount of air to move in and out of the bottle but a wet cork the pores are closed up from swelling sealing it tighter and stopping oxygen from passing into the bottle which can oxidise the wine give you off flavor and ruining. the only exception to this is a sparlking wine most maintain enough humidity in the head of the bottle to maintain corks intergrity.
also the tilt of the bottles should be no more the 15 degrees max
