Watching curling, I often see the sweeper seem to start a stopwatch before starting to sweep.
I doesn't seem to make sense that you need a stopwatch in curling. There's no difference whether the stone gets there in 10 or 15 seconds.
What is the use of the stopwatch?
I think you are seeing them stop, not start the stopwatch. They are timing how long it takes from the back line to the first hog line. This time can be compared to previous throws to see if the rock will be heavy or light.
If the time is long (rock is underthrown), they can get on the brooms immediately and try to stretch it.
Curling teams receive a specific allotment of thinking time (term used in rules) to complete the delivery of their stones.
World Curling Federation handles the timing of games in section C6. Each team is given 38 minutes for its ten ends, or 30 minutes for its eight ends, with alternative times for wheelchair or mixed doubles, and additional time for extra ends.
A team's clock runs from the instant all stones have stopped moving or left play during their opponent's previous stone, any violations have been corrected, and the opposing team have moved off the playing area or house. It stops when their own current stone passes the tee line.
Since each team uses eight stones, a ten-end game allows 28.5 seconds per stone. Teams may desire more thinking time left towards the last stone of an end or the last end of a game, so tracking is important, especially as a team must forfeit if their ends are not completed in their time.
