Wax application for most today is decorative. Some use wax like others would use the bottle neck capsule. I like to melt plain paraffin colored with crayons in an empty narrow metal cans in a water bath and dip my bottles in...just make sure your corks have been seated for at least 72 hours. Or I drop a dollop of wax on bottle and press a stamp into it to create a decorative impression. But I only do the wax stuff for special presentation, so it is waxed just prior to when I need the wine. Getting a dragonfly stamp for the dollop detail since my wines are called 'Dragonfly Wines'. Currently have a 'DW' and a 'D'. Have even considered coordinating the color of the dollop to be a visual indicator of dry, semisweet, sweet, dessert....matching my wine label colors.
Back in the day bottle necks were also wax sealed as an extra measure of protection. Bottle necks were often irregular and corks could be the same so the wax helped to ensure an airtight seal. Plus, the wax seal, which many times had an insignia impression or the like, was a verification of legit product.