openEXR is very useful for true HDR photomanipulation for the same reasons that operaguy listed above. Conventional image file formats are only 16 or 24-bit and can't contain the the full dynamic range that Poser can render and Poser will basically chop off this wider dynamic range without any input from you. If you export in openEXR format, you can tweak the levels to what you like before saving in a conventional format.