I have an image that is 320 x 482 pixels size and 72 PPI. I can change the image resolution with GIMP for example, but why would I use it if the images suits its pixel size for each display pixel size? I mean, an image changes its size in inches depending on the display density, right? So why would I want to change the PPI of an image, that doesn't make sense to me, although I understand that GIMP and any other image editors wouldn't put this feature for nothing.
PS: I tried to change the resolution of some images and compare it with the original image and apparently nothing happened but the size in inch of the image.
An image's DPI is defined by the capturing hardware and will impact it's size when printed out. Changing the resolution won't change the number of pixels in the image.