print design - What's the difference between 72ppi and 300ppi?

I created two documents with same pixel dimensions (1280x768).

• 
  

  Document 1: 72ppi

  
  


• 
  
  

  Document 2: 300ppi

  
  

When I saved both in .png, they have the same size on the hard disk. Why do I need PPI? Why can't I just use pixels?

Answer

Because in print, you need other dimensions as well as the information of the density to be able to judge if the result will print ok. Yes, as long as you stick to web you can mostly ignore it. But in print it makes all the difference.