The "alpha-channel" transparency of PNG images is very useful for creating attractive text.
Look at the image of text below.
(If you are using Internet Explorer, you will miss the whole effect. You will see the text on a gray box instead of floating above the background image.)
Alpha channel transparency should not be confused with the simpler single-color transparency. Below is the same image saved with single-color transparency:
Note that the "Aliased" text is not smoothed as in the previous case.