If you're a Harry Potter fan keep reading. If not, go read all the books and watch the movies first.
Bar tenders are like potions masters in the Wizarding world. Ingredients, recipe and technique are needed to create each brew. In my humble opinion, the classics ie; Manhattan, Vesper, Negroni, Boulevardier and the Vieux Carre should always be measured. Accuracy is quality, quality is flavor and flavor is what the guest pays for.
In Harry Potter, potions are used for many reasons. They're used to make you look like someone else (polyjuice potion), or give you great amounts of courage and luck (felix felicis) and let's not forget the oh so seductive Love Potion.
Back in the real world though, bar tenders create potions that do almost the very same things as the potions in the wizarding world. I say almost because we know that alcohol can sometimes give you great courage, but not so much luck. A mixologists brew of booze can definitely make you fall in love or more like lust. And when under the poisonous potions spell it can change the appearance of the person right in front of you with the effects lasting for up to several hours.
Just like the potions in the wizarding world, the potions at the bar can be incredibly dangerous. PLEASE DRINK RESPONSIBLY.