Why do organisms reproduce, even though reproduction is not as vital as respiration, excretion, etc. at the expense of energy?
To ensure continuity of their species.
Reproduction, compared to other life processes such as respiration, digestion, circulation, etc. is not that vital and essential to maintain life of an organism. Morover, the process of reproduction takes more energy in comparision. Despite that, It is still a crucial process as it ensures continuity of their species in the biosphere.