Holding modifier key scrolling up/down with mouse triggers macro (or another key) only once till modidier key pressed.

Usecase: Working in graphical environment (CADs, diagram software) with one hand on mouse there is more comfortable to use mod+mwheeldown to copy (give it to me association) and mod+mwheelup to paste (take it back association). Selecting area with mouse, then holding mod key scrolling down (copy), then moves cursor to new position and scroll up (paste). This approach also useful while editing wysiwyg-type document editors.