It sells the chair twice, honestly.
A colour is apply, then process, then finish, and through the middle stretch the stylist is doing nothing. Most diaries block the whole span anyway, so every colour quietly costs a salon an hour it was never using. Callra models the three parts separately and can book a cut into the processing window.
You switch it on per salon and off per service, so the bleach nobody should walk away from stays blocked end to end. Left off, nothing about your diary changes.