Why? Because your eye "reads" a photo from left to right, in the same way that we read text. It also tends to read strong lines first. So, in this original image, the eye follows the strong diagonal from the bottom left and follows it out of the frame on the top right. Obviously, it would be preferable for the eye not to be lead out of the frame in an image, so flipping it would allow the eye to run into the main focal point of the image (so from top left to bottom right) It's a small detail, but easy to change so it's worth the two extra seconds it takes.
It's so simple to do. You either need to do this at the start of the editing process - before you have any layers - or at the end of your edit after flattening all the layers.
With the background layer selected in your layer's palette, go to
>LAYER >NEW >BACKGROUND FROM LAYER
Now go to:
> EDIT > TRANSFORM >FLIP HORIZONTAL
That's it - your image is now flipped and looks like this......
A simple change, but worth doing to some images for different reasons so good to have in your editing arsenal!