Yes, but not much. They are very good at storing water. Many desert plants store water inside themselves, like cacti, for instance.
All plants need water, regardless of color.
I'm assuming you meant "why", not "what". All organisms (living things) need water to survive, not just plants. Water serves many functions in plants, including as a part of chemical reactions that...
It's just the same basic necessity that humans need...
No, while there are some plants that can get by with a great deal less water, xerophytic plants, but all plants need at least some water.
for transpiration