png as a graphic format, are well compressed, without looses and highly compressed!
You can use alpha channels, color profiles (for prepressing), include more information (keywords for searching in internet and others)
When you open a PNG in Photoshop, it opens as "Layer 1", without background, perfect to see the transparency. If you wish to save in Photoshop to PNG, first merge all layers (The background never), and select "Save as" CTRL+SHIFT+S; surelly the file as saved as a copy, caution, the file that remains open it's allways the original PSD file, close without saving (after saving as a PNG).
Is the best format in my apinion, for Internet and screen graphics