Can someone specify the extra features that an arranger keyboard has over other keyboards such as workstations, and why someone would need those features if any.

What are the advantages to a composer/arranger of having an arranger keyboard over having a workstation? Or is a workstation such as an Roland XP-80 better to have then an arranger keyboard?

I'm looking for some clarification of exactly what is an arranger keyboard. Even a web address that explains this would be great.

Thank you.