AAC (better knownm as Advanced Audio Coding) is an audio format very similar to MP3, but that featuring additional performance improvements.
Compared to MP3s, AAC files feature higher encoding efficiency, simpler filterbank and a highly optimized handling of frequencies above 16 kHz.
Even though the AAC format is not as popular as MP3, there are users who always choose AAC over MP3 since it sounds a lot better at similar bit rates.
Furthermore, the AAC audio format can also be accessed by almost all available multimedia players.
Moreover, one may rely on AAC players to easily make playlists and enjoy their preferred tracks, be they AAC files or not.