An educator trick is that drinking from a coffee/water, etc is a natural way to buy yourself a moment to rest your voice, think of something clever, etc

Are you asking why people have preferences.

It’s an old bar trick to serve the performers the drinks the house wants to sell to everyone else — except what they really serve them is water or colored water.

This is also done after the set, where if someone buys them a drink, the house gives them water and charges the person for the drink, and the performer gets a cut of the money.

That way, the performer can accept as many drinks as are offered without being unable to perform their next set.

Most are props. Watch any of those comedians that walk around with Scotch glasses and see how many times they actually take a drink.

It usually is water. Even the “beer”.

They're only on stage for an hour at most. They can drink all they want before or after, so dehydration isn't a real concern. With all that talking, dry mouth can be. The drink is just something to wet the pallet palate, to aid in speaking comfortably and clearly. For that, it doesn't mater much what it is, so long as it's wet. So it's just whatever they feel like drinking.

Standup comedy is all about personality and "vibes". What they're drinking affects the audience's perception of both. It helps to set expectations. Their drink is often part of the performance. (Mind you, comics don't always have beverages. I watched a comedy routine today and I'm pretty sure he didn't have a beverage at all. Not even water.)

Understand that this is second hand info. I'm not a standup, just an interested hobbyist

It varies, and there's a difference between club comics, touring comics, and the big names doing sizable venues.

Your typical local comic that only works one or two clubs, they're often drinking water or soda, and it's a gimmick to sell drinks they get a cut of, though that cut might be tiny. They don't get paid enough to really drink between sets, and they would absolutely be paying for any booze they have on stage.

Your touring comic, the ones that are usually doing clubs and smaller venues across a country, is usually the same basic idea, but you will run into them having something they prefer over water or soda. They tend to have longer sets, even in clubs, and only one set a night. So they don't have to worry about drinking too much and fucking their act. Not that a lot of comics actually minimize drinking, but the serious ones keep it to a minimum when they'll be on stage and save any hard drinking for after. So you'll see some beer, maybe the occasional mixed drink.

The big names, they have more freedom, and longer sets. Think someone like maybe Larry the cable guy, on up in drawing power to where Eddie Murphy was in the eighties. Popular enough to put butts in seats solo in arenas and bigger auditoriums. They drink whatever the fuck they want on stage. They tend to pick things they either enjoy, or will keep their throat and mouth in good form. Ron White was kinda well known for scotch and other whiskey on stage, but he'd be sipping at it, and the ice was diluting it over his shows. But most of those comics err more on the side of lubricating the pipes than anything else.

Why do you like coke, and I like pepsi, or vice-versa?