Maybe you would like to use the RPG-SoundMixer for background play-back, while you are working with other programs. This is no problem for RPG-SoundMixer.
The question is whether or not you would like SoundMixer to continue reacting to keystrokes or not. Imagine you are working with a text processing program and SoundMixer reacts on each keystroke with sound-effects - no one wants that! So key monitoring can be disabled in RPG-Soundmixer.
The setting "Key monitoring always active" in the "Settings" menu lets you determine how the SoundMixer should react when it not the currently active program or when you're working in another program. A check on this menu entry shows that SoundMixer is monitoring the keys. If this option is unchecked, the SoundMixer remains in the background and does not accept keyboard commands unless it is the program in focus.