The art obviously, but for me the compact-ness is a major factor.
It is my equivalent of those beautifully compact missal/prayerbooks which the Victorians never travelled without. Something compact, with scope for reflection to really lift me out of (and at the same time take me into) myself.
As if immediate contemplation or meditation were at my fingertips.
Of course there's the predictiveness but I don't seek solace from tarot cards just because they might predict something. The idea of all those symbols and that mysteriousness compressed and in my bag in a lovely pouch, is just heaven for me!