The aim of that is to stop unauthorised people drinking the contents (in particular, "servants and younger sons getting at the whisky"), while still allowing them to be on show.
Formålet er at forhindre uautoriserede perosoner i at drikke indeholder (særligt i at "tjenere og yngre sønner får fat i whiskyen"), mens den stadig gør det muligt at udstille indholdet.