What is the purpose of adding rum, whiskey, brandy, etc… to beer?
Should you be able to taste the hard stuff or should it blend well with the beer?
What are some beer recipes designed for each of the three: rum, whiskey and brandy?
It tastes good, that’s the purpose.
How much is a matter of preference. Most people would prefer it adds a little flavor without overwhelming the beer or making it taste “hot”.
A lot of people add these things to strong beers like Imperial Stout, barleywine, Scotch ale, and such as that, but it’s not a requirement. You can of course add to any style you want.
In my experience they are typically added with oak, not straight. The liquor is a great way to extract flavor from the oak or other things like cacao or vanilla beans.
Extracting flavors, and also to simulate barrel aging using cubes or spirals.
I’ve done a stout with rum soaked oak, and I’m about to do a stout with bourbon soaked cocoa nibs (no wood). In my experience, darker, heavier beers like stouts, barleywines, wee heavy etc play the best with barrels/wood. I’ve never done anything with brandy, but a lot of sour beers as well as saisons and farmhouse beers that are aged in wine barrels, so that might be good.
In my opinion, you don’t want it to be overpowering but you want to know it’s there. When someone drinks your beer you want them to be able to pick out hints of whiskey or rum etc.
Also just to clarify, it is illegal for commercial brewers to add alcohol to beer. They are allowed to have the “residue” that’s left over from empty barrels blend in with a beer, but they can’t just dump a bottle of bourbon in a keg. We as homebrewers can do whatever we want, because our beer is not taxed and not for sale, but I wouldn’t go as crazy as just pouring some in. I’d use it to soak oak cubes, or ingredients. I’ve used neutral spirits (vodka) to soak cocoa nibs when I just wanted to extract flavor.