If running navigation in the background, volume will lower when navigation has something important to say. If the navigation volume is turned down, the radio volume will still decrease and make you wonder why.
I use Waze for navigation and experience fluctuations in the radio volume as a result.
I don’t care about the scientific reasons or the mechanical reasons.
I just don’t like the “feel” of the engine shutting off when I stop, so I spent the $100
Purely personal preference.