Final answer:
Rinsing a glass with cold water before filling helps remove impurities or residue that may affect the taste or appearance of the drink.
Step-by-step explanation:
When rinsing a glass with cold water before filling, it helps to remove any impurities or residue that may be present on the glass. Cold water is effective in loosening and washing away particles, such as dust or soap residue, that can affect the taste or appearance of the drink being poured into the glass.