The need for gapless playback metadata is to permit a playback experience that matches the Artist’s intended playback experience in the original medium.
CDs, LPs, and cassette tapes all have in common that they store audio content as one or two long unbroken audio signal(s): ~60-70 minutes x 1 for CD, 25 minutes x 2 for LP, etc… If a listener starts playing at the beginning, and listens to the end, they hear the entire audio signal. The Artist can choose to designate moments within this long signal as “start of track” points. A CD player and LP turntable provide ways to jump to these start of track points, but they are just moments within the long signal. The Artist can choose to insert silence into the long signal, usually before the “start of track” point. That silence is just part of the long signal. Sometimes the Artist chooses not to insert silence, instead having audible audio continuously across the “start of track” point.
A listener who starts listening at one start of track point, and keeps listening through the silence, and then past the next start of track point, will probably say that the first track ended at the moment the audible audio stopped and the silence began; and that the next track started at the moment the silence stopped and the next audible audio began.
I imagine most listeners will say that the period of silence is not part of either track. It is an inter-track gap, and it is not itself a track. If the audio signal is continuous across a start of track point, the listener will say that the previous track ends and the next track begins exactly at that point, and that there is no inter-track gap.
Given that the original medium has one or two long continuous audio signals, and that listener model of tracks and inter-track gaps, consider that we usually rip into multiple digital music files. The audio in each track usually begins at the Artists “start of track” point, and ends at the “end” of that track. The ending might be at the moment the next track starts, or at the moment of the last audible audio of the track. Reasonable people could choose to 1) include or 2) leave out the inter-track silence. Or there might have been 3) no inter-track silence. And, there is apparently nothing in the MusicBrainz Release entry to indicate which of these three cases applies — not even whether or not inter-track silences were present, and how long they were.
Now consider a digital music player app, trying to play these multiple music files, and recreate the experience of listening to the original medium’s long audio signal. When it finishes playing one track’s file, and starts playing the next, there will likely be at least a brief silence. The player could have the next file’s audio play immediately after the previous file’s audio, but that takes special work. The player must open the next file and start reading it before it finishes the previous file. In order to make the silence be the same length as on the original medium, the player must know if case 1), 2), or 3) above apply. That requires a piece of metadata.
A couple of articles explain the listener’s view of gapless playback well:
I am thinking about this because I just added a Release to group The Ship: A Contemporary Folk Music Journey. It was released on LP as uninterrupted flows of music on each side, with track listings but no inter-track silences. I wanted to recreate this playback experience with the digital files. It turns out that is possible, in some narrow circumstances, which happily apply to me. But I wanted to include the necessary metadata in MusicBrainz, and it looks like I can’t.