Do you mean "why"?
They are the main source for all living organisms because they provide energy, relatively short term, for any creature that consumes them.
The reaction which connects monomers (polymerization) is dehydration synthesis.
The name tells you about the nature of the reaction - dehydration means 'loss of water', correct? When two biological monomers connect, one gives up a hydrogen ion (H+), and the other gives up a hydroxyl group; (OH-). They combine to form H2O - water.
