Often new writers ask "What is a piano-vocal score and why is it so important?".
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 they have developed it.
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 versatile since they are quickly edited and serve as a draft manuscript for a musical. They take less time to edit and format than a conductor score and can be changed on the spot with music notation software.
Writing a musical without a PV score is comparable to 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, the director or playwright were to deliver the lines to the cast for them to learn without written text. It will make it impossible to convey the author's intent to the cast and performers, leaving them feeling uncertain about 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.