Why the cast comes first
A locked character carries a model sheet into every panel. Without one, faces drift.
When a panel is drawn, every locked character in it is attached to the request as a visual reference. That reference is what keeps a face, a haircut, a jacket and a build the same from panel to panel.
If you draw pages before locking your cast, each panel invents the character again. They will be plausible every time and different every time.
You can draw pages without a cast — the studio will warn you and let you continue. It is a recommendation, not a rule.
Did this not answer it? Open the ? menu in the studio and choose Get support — the page you were on travels with the message.