Why do you do the things you do?
Why do they drink it, as opposed to some other cocktail?
Easy - they like the taste.
Okay, sure, but why did they try it in the first place to find out that they liked it? There's zillions of cocktails, and most people haven't tried most of them.
Probably their friend ordered them one once, or they saw someone drinking one and it looked interesting.
Okay, so why did that friend order one?
Well now we're into the question of how social trends start. Usually we just have to throw up our hands and say 'peer effects' or 'opinion leaders' or 'fashion' or some equally unsatisfying explanation.
But in this case, we actually have a very definite answer of why you drink Old-Fashioneds.
You drink them because some time in 2006, a writer for the show Mad Men decided that Don Draper, the charismatic man's man main character in the show, would drink them as his drink of choice. The show became a hit, people started asking for them, and a heretofore archaic cocktail was suddenly restored to newfound celebrity.
I would wager that out of the people who drink them, at least 98% of them would swear on a stack of bibles that they drink them only because they like the taste, and not because of a desire to appear trendy.
And yet we reach a very stark conclusion. If that writer had decided that Don Draper would drink Mint Juleps instead, there's probably a high likelihood that you'd be drinking that right now, swearing equally that you just liked them for the taste.
The alternative is that some time around 1960, people's taste buds suddenly changed such that a previously tasty drink became unpleasant, and some time around 2007 they magically reverted back to enjoying them. Want to wager on that one?
Nobody likes to think that their personal tastes are actually fashions dictated by people whom they never met. But, more than we'd like to admit, they are.