.png's have open spaces that you dont want "white" or filled in -- like quick pages, alpha's, ribbons, lots of different types of elements. Mostly, papers will be .jpg's
The .psd files allow you much more versalitily and will have the layers and shapes to move.
People whose software doesn't support the .psd files will use the .pngs and drag those shapes to their documents.
as far as keeping both, that is a personal choice. I tend to keep the .psd files for templates, but if some of the .png shapes are things that I will use often, like a flourish or swirl or frame, I might also keep the .png files.
I also keep all my .psd files for my layouts. Every few months I will burn the .psd's to disc and free up space. I keep the .jpg file, so that I can find it and figure out which DVD that .psd is on, should I need it.