There are many things available but they are also used for other things. The Tascam is a dedicated music player and rehearsal unit, its function is to play, record and let you play along with what you put on it.
It is small enough to put in your pocket and listen to music you want to work on, so see it as an MP3player, it is also small enough to put on your strap and walk about practicing via headphones or Buds...you are not tied to sitting down. And of course it fits in a gig bag or case so again you can practice where you want when you want.
Yes finding apps for phones is a good use, but do not see it as the main use. Phones run many apps and in using your power to play you may find you have no power left to use it as a phone never mind a practice device.
Look look at Irig for Apple devices, i use it with garage band and it is cool. http://www.ikmultimedia.com/products/irig/