Why do we need white ink printing?

Why we need white ink? We will have some sample to show the effect below.

The first image we print on transparent acrylic directly by only color, we can see when we put on the table, the color is not clear showing full image.

The second is what we print by only white, the white ink can fully cover the image and we can not see the table.

Then we make the third image, which is print white layer then print the color. The whole image can be seen much clear.

That is reason why we need white ink printing on many different item printing, the white ink can cover the object surface color.