It's not that exotic, you've got a child or a sleeping partner or a boss in the same room and you want to watch a mostly visual video without any unnecessary jingles or other audio cues from disturbing them.

So you reduce the volume, but it only reduced the ringer for calls, which isn't what you want.

As the sibling answer says, newer Androids can do this. One click to display the volume, a touch to see the individual controls and then slide.