There are many cases where we use byte[] in our code. In fact, it is the “rawest” type possible in Java unless you go native. Thus, byte arrays are often used to store raw data such as bitmaps, audio and various binary objects.
The previous two articles on MTR were dedicated to audio decoders, including WAV and MP3. In both cases, raw PCM data that was the result of your decoding was a byte array (which you would later write to AudioTrack).
I already mentioned in one of those articles that you should consider streaming any audio that is longer than the reasonable maximum. However, even if your data will definitely fit into the heap, in most cases you can still do better than just using a byte array. Why? Read on (relevant for non-audio byte[]s as well!)