You can use a soft synthesizer on a tablet or phone as a stand alone music instrument when you play it with your fingers on a touchscreen.

But you can also run the synthesizer with MIDI files like a sequencer.

A MIDI file (*.mid) is a digital sheet music that contains all the notes for a whole orchestra for a piece of music. There are millions of MIDI songs available on the internet for virtually every piece of music ever has been written and you can make new interpretations of these songs with your synthesizer sounds.

Many MIDI files are free (Mutopia Project), other has to be licensed.

You can also connect a (hardware) keyboard to your device with a USB cable and use the synthesizer as a sound-expander and thus produce thousands of new sounds with your instrument.

By the way, you can also record your own songs and save it as MIDI files (MIDI-recorder).