Revisions are where the money goes. A project quoted for two rounds and delivered after five is a loss disguised as a happy client, and it only becomes visible when rounds are counted.
Approval has to be recorded. A client who approved artwork in a meeting and later disputes a printed error is a genuinely expensive problem, and a written sign-off is the cheapest insurance available.
Version control matters years later. A client returning for an amended leaflet in two years' time expects the file to exist, and finding it in a folder named final-final costs an hour nobody bills.
Retainers survive on visibility. Somebody watching the month's hours tick down asks for what genuinely matters instead of everything at once, and the relationship never turns into a negotiation.