For example say that you got the following folders at a location.
FolderA
FolderB
FolderC
You enter FolderB, And then you want to go to FolderC, Now you first need to go up to the parent folder and then select FolderC and enter it.
But with the Go to sibling folder commands you can go the the previous or next sibling folder directly.
So when you are in FolderB and if you instead choose to press Ctrl+PageDown it will take you directly to FolderC.
If you have pressed Ctrl+PageUp it would have taken you to FolderA