Arrangers are really intelligent MIDI sequencers that can follow your keyboard input and play back accompaniment accordingly. This give you the advantages of a sequencer except it follows you so you have realtime control of the accompaniment. MIDI sequencers would provide similar backing but you would always have to play to them rather than them following you. With arrangers you can adlib, repeat choruses and extend solos which wouldn't be possible with a fixed length MIDI sequence. Arrangers really are state of the art MIDI sequencing using all that is good but not at all confined by the restrictions of a MIDI sequence.