Is that what the question asked?
As I understand it, the question is actually asking what the biological basis is.
The short answer is that creatures that don't live in water can't really manage external fertilisation, like many sea creatures do. This just pushes the problem back one step: Why reproduce via fertilisation?
And the answer to that is that it deepens the gene pool. In a creature with 22 chromosomes then, assuming no crossovers, there are (2^22)^2 genetically possible offspring, as opposed to the 1 genetically possible offspring provided by asexual reproduction.
Deepening the gene pool protects our species from harmful genetic mutations, as well as from diseases which target monocultures.