Bars specify ice as precisely as they specify spirits, and some invest more in freezing equipment than in glassware. The reason is that ice adds water to the drink at the same time as it removes heat.
Chilling and dilution are the same event
Melting absorbs a large amount of heat, and that absorbed heat is what cools the liquid around it. The water produced stays in the glass.
A cold drink is therefore always a diluted drink, and a recipe that tastes correct at the right temperature has been written to include that added water.
Chilling a drink without ice, then serving it over ice, produces a different result: less dilution, higher intensity, and often an unbalanced finish.
Surface area sets the rate
The same mass of ice melts far faster when it is crushed than when it is a single block, because melting happens only at the surface.
Crushed ice therefore cools quickly and dilutes quickly, which suits drinks built on strong flavours meant to open up as they thin.
A single large cube in a spirit-forward drink does the opposite. It holds temperature with minimal water, which is why the format is used for drinks sipped slowly.
The temperature of the ice itself varies
Ice straight from a cold freezer is well below freezing point and takes time before it begins melting at all, so a drink shaken with it chills faster but dilutes less.
Ice that has sat out has a film of water on its surface and starts diluting immediately. Bartenders call this wet ice and generally discard it.
This is also why the first drink of a service and the tenth can differ noticeably when the ice well has not been managed.
Clarity is mostly a signal, not a function
Clear ice is made by freezing directionally so that trapped air and dissolved minerals are pushed out rather than locked in the middle.
The visual difference is obvious, but the practical difference is smaller than it appears: melt rate is governed far more by size and shape than by clarity.
Cloudy ice does fracture more readily along its internal flaws, which matters for large formats meant to survive a long drink intact.
Ice is a logistics problem before it is a craft one
A busy bar consumes an enormous volume of ice, and machines produce it at a fixed rate that cannot be increased on a hot evening.
Storage takes freezer space that competes with everything else, and ice absorbs odours from whatever shares that space.
Bars that hand-cut large format ice do so in advance, which means dedicating labour and freezer capacity days before the drinks are ordered.