Vegetables are essentially water in fancy packages. Most vegetables are over 80% water; some are close to 95% water. It costs money to put the water in, and it is a direct loss in yield and monetary return if that water comes out.
Moisture loss results in wilting, texture changes, poor flavour and spoiled appearance. However, too much water can also be a bad thing, increasing rots and disease, causing splitting and discolouration in some products, and generally degrading quality.
Getting the right balance between avoiding moisture loss by keeping humidity high while also avoiding products getting wet is one of the challenges of postharvest management.