Symmetry in one form or another is just very common. Think of man-made designed machinery – there tends to be symmetry (often bilateral and/or radial). So symmetry is actually really good design.
The fact that we have two eyes spread on the same level on our face helps us to gather the depth perception of the environment and helps with the amount you can see (as two eyes help to eliminate blind spots) – so that is a good design feature.
For 4-legged creatures, 4 legs being bilaterally symmetrical helps in the back and forth trotting/running motion. And 4 legs is advantageous to 3, because if you lift one leg, you can easily balance on three (think of how tripods are easier to set evenly than 4 legged chairs – or something with only 2 legs depends greatly on balance to stay up – otherwise it will fall over).
And the interesting thing about symmetry – is that an octopus with 8 legs can be split symmetrically in half (bilateral) and is radially symmetrical as you turn the octopus around the center of its body. But if the octopus had 7 legs, it would still by symmetrical bilaterally (the line would go through one tentacle and across the rest of the body) and radially.
I did a little more research and think about this:
“Imagine if animals like frogs, birds, cats, or humans didn’t have their two-sided symmetry. Birds might have a hard time flying with one wing. Frogs might hop in circles.” Read more here: https://askdruniverse.wsu.edu/2018/03/12/why-are-animals-symmetrical/
See also: https://en.wikipedia.org/wiki/Symmetry_in_biology
Some might say that symmetry came about early in evolutionary history and so has been like that ever since, but since there is a lack of solid evidence for evolution, I would instead say that being symmetrical is 1) very common and 2) very good design – evidence of a Creator.