In my previous post I meant exactly that...

Anyway, the reason to use an arranger is the simplicity of making a niche style(s). Their music has some basic "forms" or "bases" or "paths" and the rest is built ON that. They start with song A rhythm X and maybe 5-7 songs later they change to rhythm Y, depending on what the crowd wants. And so on and so on, until the early morning. They probably wouldn't do that easily in a synth.