As a musician who likes to play an instrument accompanied by e.g. a piano, you usually play your MIDI files on your computer which is connected to an external MIDI device (e.g. a digital piano). Having a phone which supports USB OTG and a USB-to-MIDI adapter you can use this app to play your MIDI files from your android.
Playalong provides all functionalities you need for typical play-along: you can change the speed of the playback, mute given tracks, transpose your songs (upgraded version only), or change the instrument's voice (upgraded version only). Playalong also allows you to define playlists which makes you able to sort your MIDI collection.
The visualization of the current key as well as the current meter, and having a metronome as well as the current measure displayed make this app and your device the perfect accompanist whenever your real-world accompanist(s) is/are not available to join you for a jam-session.