The point is to give a more convenient mapping environment by having all the timing changes carried over to all difficulty by a simple click of a button instead of having to make changes to each of the difficulties separately. Everything, including Kiai time, sound sets and volume, will be carried over.
Basically it is like this: (the numbers are only examples)
(1) Before changing
[Difficulty 1] BPM: 180; Offset: 1500
[Difficulty 2] BPM: 180; Offset: 1500
[Difficulty 3] BPM: 180; Offset: 1500
(2) Some changes made to timing Difficulty 1
[Difficulty 1] BPM: 120; Offset: 1503
[Difficulty 2] BPM: 180; Offset: 1500
[Difficulty 3] BPM: 180; Offset: 1500
(3) Click the "copy timing to all difficulties" (or whatever it will be called) and all difficulties are changed
[Difficulty 1] BPM: 120; Offset: 1503
[Difficulty 2] BPM: 120; Offset: 1503
[Difficulty 3] BPM: 120; Offset: 1503
[Difficulty 1] BPM: 180; Offset: 1500
[Difficulty 2] BPM: 180; Offset: 1500
[Difficulty 3] BPM: 180; Offset: 1500
(2) Some changes made to timing Difficulty 1
[Difficulty 1] BPM: 120; Offset: 1503
[Difficulty 2] BPM: 180; Offset: 1500
[Difficulty 3] BPM: 180; Offset: 1500
(3) Click the "copy timing to all difficulties" (or whatever it will be called) and all difficulties are changed
[Difficulty 1] BPM: 120; Offset: 1503
[Difficulty 2] BPM: 120; Offset: 1503
[Difficulty 3] BPM: 120; Offset: 1503