A common misconception is that they are meant to manage layers. This is not really the case (it works only for layers in “Normal” mode).
They are really like parentheses in mathematical expressions (where layer blend modes are operations and the “opacity” of the layer is a coefficient) .
This can be useful; compare for instance PadDavid’s excellent tutorial on freaky details where you have to restart from scratch and this simplified version where you can tweak things.