Encoding – Why are the indices of cmpctblock and getblocktxn messages calculated as differences?

If you want to send transaction index cmpctblock or getblocktxn Messages are displayed as differences. For example, if you want to send indices 0, 54, 65, 67, and 1236, they will be displayed as 0, 53, 10, 1, and 1168.

Why did this happen? Why are differences passed as well as indices? Is this due to some optimization? Or is it for some other reason?