Different browsers show the same images differently. The viewers' screen sizes differ, as do their skills in using a computer. Your site has to be reviewed by several different browsers to see and correct the differences. The clarity of the image is rarely as good as it is on the printed page. Allowances must be made for these differences in designing a site.
Some type faces are more readable than others, for example.
The use of color is freer on the computer, and doesn't add to cost. Add color to the printed page and your costs go up dramatically. These devices are relatively easy to put on sites, and if used tastefully, add to the interest and attraction. The content, look, design, and colors of a site can be changed at will. Obviously not so in other media.