Often, during the first two rounds when there are alot of golfers, they will send groups off of both #1 and #10. Let’s say they are playing in groups of three. Once 22 groups of three (if you factor in par 5’s where there could be 2 groups playing the same hole) start you have filled up the course and nobody has finished 18 yet. That’s fine if you have only 66 in the tournament but this week in the US Open there were alot more than that!! Then what?
They do it twice. As your 66 players finish, another group of 66 (or whatever is left) plays. On Thursday, half of the players get a time in the first group, and half get a time in the later group. Then on Friday they switch, so those who went out early the first day will go out late on the second day.
They do it this way because they can finish faster. If a round takes 4 hours, then everyone’s done 4 hours after the last tee time. With split tees like this, the last tee time is only half as many groups after the first tee time.