Arrangement is a part of project, so even in case you don’t use arranger you can write some notes (or may be a poem 
) in it about project for later reuse or when you plan to share your project with other people. Also, if you sing songs you can write lirycs like in karaoke to be sure that you will not forget it while worrying during performance.
Arranger is a very helpful especially while using too much external hardware. It helps to concentrate on sound and tweaking synths instead of worrying about banks/pattern changes or muting/unmuting tracks.
