Benefits of this type of design?
Some devices put out a high voltage signal. If you don't need it, then you may waste dynamic range by keeping it cut back. Your noise floor could intrude on some systems in other words. Imagine your amp clips with a 2 volt input, but your device puts out 8 volts. You'll have to reduce everything by at least 12 db while your noise floor isn't reduced. So you have lost 12 db of dynamic range available.
By making it work to a nominal standard of only 2 volts that is enough for most gear. Yet if needed, you add analog gain. Doing that raises both noise levels and signal levels though if done well the dynamic range available from highest to lowest levels will hardly be effected. On a system that needs the higher input everything will be proportioned properly to keep noise levels low at the resulting output. Or in other words if your system needed 8 volts to clip, this optional gain stage will get you there without compromising results for high levels or modest levels.
Such an arrangement lets the device work with good gain staging (which helps available dynamic range and keeps noise levels low) over many more systems instead of having to be carefully matched with everything else that follows.
Now some of the subsequent test results indicates this particular device may add noise along with gain or may raise distortion while adding gain. That isn't good. It indicates someone didn't fully think out the design. Or in this case probably a result of trying to run on 5v USB power.
Headphones are the default for many now.
