Our insides are not as symmetrical as our outsides. Is this true of most animals? Is there some reason for this if it is?

It is certainly true for all mammals since our internal organs are non-symmetrical. I not sure there is (or needs to be ) a reason for this. Certainly symmetry is massively advantageous when walking/moving on 2 or 4 legs/paws and I can't think of any survival or other advantage of having symmetrical internal organs.