Hello, main advantage of fade in/fade out is, that you get rid off these small little background/ambient noises (when opening your mouth: smacking with your lips, spitting-sound, clicking with the tongue at the velum, ...) partly you can't prevent. In Audacity you can see it as small little peaks before and after the actual recording. If you're aware of it you can hear it too. Of course not every recording has one or more peaks, I guess maybe 20% . But it don't harm if you fade in/out in the other cases too. So it's a quality feature that helps to improve my recordings.