Clear, big ice melts more slowly in the glass, but more importantly it's a vast aesthetic improvement that customers go crazy for. Big cubes are perfect for Old Fashioned-style cocktails that are very popular these days; in fact the cubes have helped the trend grow. Some customers just order the cocktails for the ice - you'll hear them asking for "the one with the big rock.
The only thing more elemental to The Cocktail than ethanol is ice. The cocktail at its core is a preparation of or a tempering of a spirit and ice is the first tool always reached for in that toolbox. Every cocktail is balanced a little bit differently and thoughtful use of ice can make all the difference between a good cocktail and a great one. Large, clear, perfect ice gives one the absolute most control over temperature holding and dilution rates. It can ensure that a perfectly balanced cocktail can stay perfectly balanced from the first sip to the last drop. In addition to that, it's beautiful. The guest's perception of the value of a cocktail or a whiskey pour goes up significantly when they are presented with a perfect piece of ice in their drink. I'm incredibly lucky to have a vendor like Fat Ice to help me with my needs. I simply don't have the infrastructure to run a Clinebell program in-house and just freezing with small molds yields really unimpressive results so I choose to source my large format ice and it's been a consistently good decision.
Ice not only dilutes, it also chills your cocktail, which along with proper balance make a great drink. Having the best quality ice to start with ensure you can control the overall outcome of your cocktail without worrying about compromising balance and presentation, rather enhancing it.