Mirroring projection painting

In the UV menu of the UV Editor, there is a ‘mirror’ command that mirrors the UV coordinates on either the X or Y axis. I would assume this would help the projection painting of two sides of a symmetrical model; but once the coordinates are mirrored, the new mirror only appears to be active as the current UV work. There is no way to use this mirror in any other UV orientation. One would think the mirror command could somehow allow the transfer of the mirrored coordinates into a new UV image - or else what is the purpose of mirroring?

For example, I have a model that I am projection painting the left and right sides. The model is perfectly symmetrical. I paint the right side, and think if I mirror, I should be able to apply the mirror to the left side and the work will perfectly match on both sides. But once mirrored, the new mirror (left) only is in effect on my current, ‘right’ painting. If I try to open a new ‘left’ UV image, there’s no way to use the mirrored UV. So painting the left and right sides has to be done separately, which could cause errors (unless both sides are painted EXACTLY the same), plus is more work and time. Is there something about UV mirroring I’m missing?

I still haven’t found a reason or use for this mirror command and it continues to be a mystery if anyone would like to comment…