Often new writers ask "What is a piano vocal score and why is it so important?".
First, we need to define what a "piano vocal score" is and what it isn't. A piano vocal score ("PV" for short) is a notated piece of sheet music that contains the essential elements and bare bones of a musical: vocals, lyrics, and a strong piano part. Sometimes they can even include bass and drum cues depending on the style of the music and how far along it's been developed.
The PV score is used in the development of a musical, theatre rehearsals, call back sides/cuts, and running a show with no orchestra present. PV scores are very versatile in that they can be edited quickly and can serve as a draft manuscript of a musical. They take less time to edit and format than a conductor score and therefore can be changed on the spot with notation software.
Writing a musical without a PV score is like writing a play without a script; you can't do it. At least, you can't do it well. Imagine if during a 29 hour reading that the director or playwright were to dictate and feed the lines of the play to the cast for them to learn with no visual or written text. It will be impossible to effectively convey the authors intent to the cast and will make it impossible for the performers to feel confident in the material.
Once the framework of the songs and music are notated, you can then formally copyright your intellectual property (IP) with the United States Copyright office by submitting your PV score. You can also submit your finished PV score to festivals, theatres, licensing companies, and contests to be reviewed and allow others to understand the intent and artistry in your writing.
After a PV score is finished, the author(s) can then decide to have their musical orchestrated and fleshed out. But only once a blueprint of your show, or PV score, is written can these next steps occur.