P.S. And it's also for situations when you want to force the direction of the sync, i.e. when you want to ignore certain changes for certain files or folders on one machine and overwrite them with older versions from the other machine.
Just to clarify when this might be useful. Image you have the same already synced copy of a database file on the desktop and on a laptop. Then you do a lot of changes on the laptop to that file.
Then you reopen the same database application later on the desktop, to check something without changing anything. However, even that would have modified the file, as lots of database type apps might modify a file when you just open it to look at it.
Some cloud services might pick the desktop version as the latest version and overwrite the laptop version that has the important changes. Allway Sync allows you to review the files and force the sync in the direction in which you want it to happen, not which file was changed last.