.PNGs are very important to Poser users!

When you open a Poser Library all of the thumbnails are genreated by .PNG files.

Also once you've rendered a person if you want to export the render of the person, without the rest of the image(the Poser scene's background;) for composition here in Photoshop onto say an actual photograph, then .PNG is the only way to export an image with Transparent pixels(the Poser scene background at that point.)