Add a Time Tracking column to any board and every item gets a start/stop timer plus a manual entry option for time logged after the fact. The timesheet view rolls all of it up by person, by board, or by date range, which covers basic internal visibility, who worked on what, and for how long, without installing anything extra.
Higher plan tiers add more reporting depth on top of the raw numbers: breakdowns by group, exportable timesheets, and the ability to see tracked time alongside budget columns on the same board.
What it doesn't do natively is turn a billing period's tracked hours into an actual invoice. The timesheet shows you the number. Getting from that number to a line-itemed bill in your accounting system is a manual export-and-rebuild step for most teams, every time.