Here is a screenshot taken of the Photoshop optimization tool. The image of the fiddlehead greens is shown here twice big as it acually is to emphasize the differences in the quality of the images. If the resolution of your monitor is high enough, you can see that the original in the upper left, the image at 100% quality in the upper right and the image at 60% quality in the lower left hardly differ, yet the file sizes are dramatically different. the original is 19K, the "best" jpg is a little over one-third as big, and the 60% jpg is 14% as big. That's a lot of savings in download time for not much sacrifice of quality. But as you can see, there is a limit. The image on the lower right was saved at nearly the lowest possible quality. Look closely, and you will see the "artifacts" (unwanted shifts in color and tone) at the edges of the light and dark areas of the image.
