Why do biological systems need water?

Hint: Water contributes upto for $60 - 75\% $ of a person's weight. Dehydration can occur with a loss of merely $4\% $ of total body water, and a loss of $15\% $ can be fatal. Similarly, a human may go a month without eating but not three days without water.

Complete answer:
 In any biology system, water plays a crucial structural role. Water tends to fill cells visually to help them maintain their shape and structure. It is similar to putting air in a balloon, the water inside many cells (including those that make up the human body) exerts pressure that opposes external forces.
When we look at the function of water at the molecular level, water permits everything inside cells to have the proper structure. One of water's most significant functions is to maintain form or size or shape, which is essential for metabolic activities.
There are a number of functions which are performed by the water which are important for maintaining living organisms.
Water is used as a mode of transportation of different chemicals, organisms, and other objects. Water basically acts as a medium for transportation. Plasma is made up of $90\% $ the water as a main constituent. Water acts as a medium to distribute different things throughout the human body, including blood cells and hormones.
Water is also required for intracellular and intercellular transportation.
Water is used as a universal solvent because it dissolves a number of substances like sodium chloride and other salts present in our body.
Water is used as a metabolite in chemical reactions that occur inside the body.
Water is responsible for maintaining the mobility of sperms in males.

Note:
In absence of water, spermatozoa cannot move by using their flagella. Hence the process of fertilization is not possible in dehydrated conditions. Water also acts as a medium to remove toxic products like nitrogenous substances out of the human body.