Cork is a unique and natural product derived from specifically grown cork oak trees and therefore there is always the chance that bacteria can carry through. No matter how sanitised the corks are, the pores within the product are susceptible to holding onto tiny bacterias (nothing dangerous in regards to your health).
Originally it was customary to decant wine before service as it was stored in large amphoraes rather than small bottles, making it difficult to serve wine at a table. Therefore with decanting, the wine could be transferred to a smaller jug or pouring vessel to be carried around by the wait staff. Many years have passed since the 17th century yet it is still common practice to decant certain bottles of wine before drinking. No longer due to ease of pouring (as a bottle of wine is quite easily held in hand) but to ensure the wine is at its peak tasting potential for the drinker.