We often cook vegetables by boiling them in a pan of water. This causes much of the goodness to leach out into the water during the cooking process. The vegetables will inevitably lose some of their essential minerals and vitamins on the way to your plate.
They may even lose some shape and colour, making them less appealing to eat, less tasty and less filling.
Whilst you can recapture some of the nutrients by using the cooking water for sauces and gravies, those in the steam are lost forever, perhaps transferred instead to an oily film on your walls and kitchen cupboards.