Slower metabolism so less vital heat. Prevent wounds, abrasions, bruising, or punctures at harvest. Bruising from dropping the produce or dropping something on the produce. Onions will show internal bruising damage if dropped 30 cm. Throw out culls to prevent disease from spreading.
Pathogens can contaminate produce through wounds.
Give the produce a smooth ride, not bumpy. Optimum temperature is the most important factor to maintain quality. A continuously flowing cold water bath conducts heat away. Do not hydro-cool eggplant, garlic, or onion. Amount varies with crop.