A lot of Android smartphones have a 3.5mm audio jack built in for playback to headphones. While this is standard to almost all phones and widely used, it does not always provide the best audio experience and can sometimes seem a bit lack lustre. Lower end phones may seem like they do not have enough power behind the music, making it sound flat or tinny.
This allows your phone to play high quality audio while still being portable and gives you much finer volume control.
Please note: Android phones running version 5.0 “Lollipop” or above will natively support audio over the USB port providing the phone has USB On-The-Go and the manufacturer of the device has enabled the feature. High quality formats such as Apple Lossless or FLAC may only be played providing they are supported by your music player software.