Time to fill measures the role rather than the candidate. The clock starts the day a job requisition is opened and approved, and stops the day someone accepts the offer. Because it begins before anyone has even applied, it captures the whole effort of sourcing and the difficulty of the market for that role, not just the speed of your process once candidates are in it.
That makes it a useful companion to time to hire. Time to hire tells you how efficiently you move an individual through your pipeline, while time to fill tells you how hard the role is to fill in the first place. A long time to fill with a short time to hire usually means the challenge is finding candidates, not processing them, which points you at sourcing rather than at your interview loop.
Tracking both over time is how a team learns which roles need more sourcing effort, which stages slow things down, and how realistic a given hiring plan is. On its own, a single number hides all of that, so time to fill is most valuable watched as a trend and broken down by role.