Bonus tracks can be the best tracks on an album, whether they are 'secret' unlisted tracks (like the final thrashy song on Nevermind that would jolt on after long minutes of CD-spinning sibilance), or outtakes and live versions, or the mono versions included on a reissue for the delectation of the thorough and the obsessed. They don't have to be coherent with the rest of the album's content. They don't have to be justified or explained. They are lagniappe, offered in goodwill, amusements and diversions. 

Here are some bonus tracks:  

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Drawings, mainly of dogs and other animals.