Enzymes are the catalysts that alter the rates of chemical/biochemical reactions thereby enabling the sustenance of life.
Enzymes bind reversibly with the substrates forming enzyme-substrate complex. These complexes are then converted to the product and the enzymes are regenerated.
i.e., E + S `harr` ES `harr` E + P
Enzymes operate by lowering the activation energy of the reaction by providing alternate pathway for the reaction. This saves the energy required to overcome the high activation energy levels and also increases the rates of the reactions. Without the heightened rate of reactions, the human body (or animal bodies) will not be able to carry out the chemical reactions at a fast enough rate to sustain life. It is estimated that enzymes speed up bodily reactions by about 1 million times as compared to body without enzymes.
Hence, enzymes enable our survival by increasing the rate of reactions.