Such is the confusion that humans experience when first learning about genetic engineering in their past and seeding planets in general. The genetic engineering practices they learn about do not reflect, on their face, the history of these practices. Life first begins as a natural result of the processes that occur on all life bearing planets - the warm soup of oceans, filled with the simple genetic components that can and do linkup in an endless variety of combinations.
Humans are aware that the DNA on Earth is all composed of four simple building blocks.
In other parts of the Universe such components differ from those on Earth, nor is four the magic number. Big bangs occur periodically in this or that section of the Universe, and life forming after a big bang evolves slowly. In that same section of the Universe numerous worlds will eventually produce, during the natural course of evolution, intelligent creatures capable of offering a home to forming entities. Seeding planets and genetic engineering is eventually done within a section of the Universe for the same reasons that human couples long to have children - for more company and the pride that parentage brings. The process of evolution is expedited, bumped from slow motion to fast, and the number of worlds eventually becoming life-bearing planets is increased.