With existing formats, there are substantial cross-platform differences with image viewing. The images will be displayed, sure, but what people see will be very different.
Create a graphic on a Mac, say, and balance it to look just right. This picture of two children, for example.
Trouble is, it will look dark and contrasty on a PC, because the default gamma correction is different from that on a Mac. On an SGI workstation, however, it will look pale and washed-out because there the gamma is different again.
 On a PC |  On an SGI |
The problem is that your browser has no idea where the image was
created or how it was originally displayed, so it cannot compensate
for these differences. What is needed is for the authoring tools to
include this information, which is readilly available to them. But
existing image formats have no way of storing this information.
PNG stores the gamma value used by the source platform which created the image, in a standard place in the file which browsers, image viewers and authoring tools know how to read and adjust for. So the gross lightness and contrast differences we have just seen are compensated for automatically, without the image designer or the reader having to make any adjustments themselves.
 On a PC | On a Mac |  On an SGI |
Not identical, but a lot better.