Why? Wait until a bloody sit down! Sit downs happen after the 3rd, 5th, 7th, 9th, 11th and 13th games of a set, as well as always happening at the end of a set. It's not rocket science. It's been the sit down rule since the start of 2000.
The old rule before 2000 was harder for fans. From 1974-1999, sit downs happened after the 1st, 3rd, 5th, 7th, 9th, 11th, 13th etc. game of a set. Sit downs only happened at the end of a set back then if the set ended 6-1, 6-3 or 7-6 (odd number of games). If a set finished 6-0, 6-2, 6-4 or 7-5 (even number of games), the players would remain standing and play the first game of the next set before sitting down, and the audience never seemed to understand that last bit back then, always moving at the end of a set.