Some businesses sell things; services businesses sell time — and time is the easiest product in the world to lose track of. Time Tracking turns logged hours into approved, priced, billable facts: work is organised into projects for a customer, people record time entries against them, the time is approved, and approved time becomes a draft sales invoice through the ordinary sales flow (Chapter 3) — no parallel billing system to reconcile.
Every billed hour can prove itself: entry, approval and invoice line are linked, so a client who questions an invoice can be shown the day, the person, the hours and the description the charge came from. And time that has not been billed yet is never invisible — it sits in the Unbilled WIP report (§9.9), valued, waiting to be asked for.