The 72dpi figure is nothing more than a bit of meta-data in the file for a suggested output size. This figure was standardized on the original Macontish copmputer IIRC to reflect pixels per inch for document layout. The dpi number can be changed in editing programs but means bupkis as far as image quality goes so someone's "300dpi" image isn't better becaus of it.
Many progams will use this figure for print scaling if you choose to use it. If I take a 5mp image and try to print at 100% I'll get a tiny crop of a 26.66x35.5 image. If I use the resize funtion in Paintshop to change the image to 240dpi without actually changing the size of the bitmap I'll get something that will print 8x10.66 at 100%.