Becous of genetic diversity. If you outo reproduce yourself you basicly make copys of your self. If you mix your genes with an other you make a new unique individual.
This is better for a species as a whole since this is the stuf that makes evolution happen. One critter with good genes comes to gether with an other creature with good genes making a baby with even better genes (if everything go's richt that is). And the even better one go's along and spreas its genes and makes even better babys. And so on and so forth.
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