One of the many great ways leaves can be used when outdoors is to catch and collect water, especially in areas where the ground water or available water sources might be undrinkable.
Rain water is really one of the easiest forms of drinking water. Often, you can drink it without boiling or purifying, so if you can find a way to harvest it, you're good to go. In some situations, you might use a tarp to harvest the rain water, but with some ingenuity, leaves can be set up in a way (like above) to collect drinkable water.
It's best to still filter this water if you plan to use this as your only source of water and you have a filter (or boiling capabilities). However, in a true survival situation, it's OK to just collect and go: you'll be fine, your need for water is much higher than your risk of contamination in most cases.