With arranger you have all your rythms to use for playing live or composing your own songs instead of relying on the drummers, etc. If one does not use this feature then I do not see any reason of owning arrangers. Rythms, fill-ins (drum fills) are what distinguish arranger and the strongest reason of choosing it. If a person always plays in a band (group of people that plays rythms such as bass and drums), it's probably better to use non-arranger like synthesizer that solely focus on sound quality and sequencing.

Some arrangers (high ends mostly) also have built in synthesizer capability where you can modify the sounds to your heart content like in KORG PA80 for example. I personally never had time fiddling around with those. I just use the preset sounds from the factory.