Singers and soloists who have two jobs and a family to support will find it difficult to get musicians together for rehearsals. Centres like Toronto have a wide assortment of musicians who can read. A lead sheet can provide a way for musicians to see where your song is going and in a single page can follow the structure and perform without any mistakes.
What does a musician see in a leadsheet
1) The structure of the song
2) The Key, the timing, the tempo and feel of the song
2) The pattern of chords in convenient easy to remember layout
3) A Bass player or keyboardist can see the Bass note if slash notation is used in the chords
4) A guitarist or keyboardist can see when the vocal melody is present and avoid playing on top of the words or the soloist.
5) If a musician cant read the staff they still have the chord symbols to use.
So a leadsheet will convey a whole lotta stuff that might be hard to explain to everyone
To be more complete the band can use one sheet for nearly everyone unless there is a transposing
instrument like a trumpet, sax or clarinet in which case the sheets can be prepared for if needed.