For my first business class flight last year, I had read all about champage, Kir Royales etc on BA and tried it out based upon that. Champagne isn't something I would normally drink and paying £30 or whatever for a bottle from a supermarket is more than I spend on a night out.
In my case, it's just following the flock rather than something I normally do i.e it seems to be the done thing so why not be part of that.

Did get a bit of a headache from champagne and altitude so either I'm not used to it or just a lightweight compared to some people here who will easily drink their ticket price in Krug