It used to be that when you saw a bartender using a jigger—a small measuring cup for drinks—you assumed he was either a new bartender or micromanaged by his superiors. Why else wouldn't he just "count," whereby you pour the bottle and issue a count in your head equivalent to the cup-measured pour?
Worse yet, the assumption would be that the bartender was being cheap. Nothing erodes customer confidence quicker than an ungenerous bartender, and the dual-sided metal cups might well have been the mascot of this stinginess. Of course, times change.
With the rise of bartending culture and classic cocktails, the jigger has become the mark of precision and care in crafting cocktails. We have now come to expect our Manhattans and Martinis will be made with some mathematical formulae—two to one or three to one—and measured to order. This is good news for cocktail purists.
It's been pointed out by industry luminaries that making cocktails resembles baking more than savory cooking. With baking you must measure ingredients in advance of cooking because after heat has been applied the product changes dramatically in texture and flavor. The same thing happens when we shake the cocktail with ice.