Track at once is probably used as the default because it is less likely to produce a buffer underrun if the user’s burner does not have buffer underrun protection. The developer’s have no idea of the user’s intended purpose or hardware. Older burners may not support Disk at once. Track at once is probably used as a “safe” default although it is generally inferior to Disk at once.
Commercial CD’s with a collection of different songs usually have 2 seconds of silence between tracks by default in order to separate the songs and prepare the listener for the new selection. This is just a general rule that doesn’t have to be followed, as some tracks are supposed to run continuously into the next selection without interruption. Classical music tracks frequently have no silence between tracks established for movements of the music selection.
DAO does nothing whatsoever to the 2 seconds of silence (or non-silence) as the case may be. It just records the music exactly as was recorded whether there is a silence between tracks or not. The laser remains on for the entire burn.
On the other hand, if TAO is used when recorded tracks run together, it will insert a 2 second “gap” of silence between tracks when the laser turns off for this period. This interrupt the music flow and creates an annoying gap and sometimes pops & clicks between tracks. If the tracks don’t run together, the 2 second gap with TAO might not be noticed. Because TAO has this behavior, some think that DAO “eliminates the 2 second gap”. This is not true. It might recover gap errors introduced by TAO, but DAO just records what is already present.