Evolution

The  symmetry can be explained by common ancestry tracing back to an early ancestor that had a symmetrical body plan. So natural selection obviously has some role in preserving this symmetry throughout evolution.

The natural selection explanation is that bilateral symmetry provides some benefit  and so there has never been much pressure to change it. It is useful because it is economical during development of the embryo. The same genetic instructions for building the right hand, will also build a left hand. The instructions for one eye can build the other. Instructions for building the left-side of the rib cage, also builds the right.