The Use System Player feature was developed it allowed greater control over the volume of music, ducking etc if you played the music internally within the app by disabling the system player.
However, this was before Apple had released Apple Music. Due to the copyright protection Apple use with their music service Seconds cannot play their music internally and instead must delegate the music playback to the system player.
When using the system player is it not possible to control the volume of the music anymore, and ducking is provided by the system and is less flexible.
As more and more people use Apple Music there is less and less reason to keep this feature in the app and it will be removed removed in the future instead always using the system player.