Blend modes do not always require there to be transparency, because they deal with only certain elements in the picture, except for screen, which makes everything lighter and lighten. Overlay is like lighten but bumps up the saturation also. Multiply can be used to drop color into a black and white drawing and make it seem like it was always there, it acts on the layer(s) under it.

Blend modes such as screen continue to add more lightness if duplicated.

Saturation, obviously works like an adjustment layer to increase saturation/decrease saturation.

Hard light makes colors extremely vivid and looks like you let Andy Warhol get into your computer. Except for cartoon effects, it is one I rarely use.

Soft light limits harsh extremes of color. Can be useful if you have beefed up saturation and then need to soften some elements in the picture.

Exclusion is almost like invert, difference is like cross-polinating multiply and exclusion.

Add is like screen but on steroids, and likewise subtract is the opposite of add and does what multiply does but much more so. Add is often used to knock the grayness out of lens flares and starburst, whereas subtract and multiply could be used on very over exposed photographs to bring them back to what they should be.

Blend modes can ignore or see through 100% white and black and deal with just those parts of the picture to emphasize them, that is why some of the layers were used as though they were adjustment layers.

I am not sure if I always understand what I am doing. I just flip from one to the other to find the best mode to do what I need done.