What's in it for the Designer?
The benefits of fully supported PNG are many but the most apparent to Web Designers is their support of Alpha Channels. Alpha channels are an 8-bit channel attached to a 24-bit graphic effectively making it 32-bit. Alpha channels, instead of defining color, define variable levels of transparency... 256 levels of it in fact. This is quite different than the transparency that can be incorporated into the GIF format. Each pixel in a transparent GIF are either completely opaque or completely transparent. In other words you can either see the pixel or not see it depending on its transparency assignment. This simplistic interpretation of transparency didn't allow the designer much leeway when designing graphics meant to appear on multiple backgrounds.

A case in point; what if you had a capsule shaped red button with a white outline around it (see examples below) and a requirement for that button to appear on multiple pages with different background colors? Wouldn't it be convenient if you could make the button once and use it over and over again? Most certainly, but saving the button as a transparent GIF and using it over and over again presents a few problems. The most apparent is a phenomena known as "fringe".