What is the importance of metabolism, particularly during periods of rapid growth such as germination.....?

Question

....when the energy stored in the endosperm must be mobilized to provide ATP required for growth?

Answer 1

Metabolism is the set of chemical reactions that occur in living organisms in order to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down large molecules, for example to harvest energy in cellular respiration. Anabolism, on the other hand, uses energy to construct components of cells such as proteins and nucleic acids.

Clearly starch or lipids stored in endosperm has to be mobilised  in order for germination to take place - this is the role of metabolism which results in the formation of energy (ATP in respiration = catabolic) and the synthesis of new plant material as the seed germinates (=anabolic).