This slider is specialized for setting a low volume.
It has better operability than the control center.
The volume buttons on the side of the device have large width steps, Even at the lowest setting, the sound is rather loud. You can set lower on this app.

Caution.
At very low volumes, the numbers may differ from how you actually feel.
This app is designed to adjust the volume of the device's speaker. It may not work well in other ways.