SFFS uses a database to store the files in the destination.
Later, it checks that database instead of re-reading all the files.
The most obvious use case is when you have just finished the synch profile a few minutes ago. Then you change some other file and would like to synch again. Having a cache of the destination filesystem (I mean the folder you are synching) would save the time to re-read from disk again.
If you download the 30day shareware of SFFS, you can go to Advanced settings > special > check box 'cache destination filelist'
You may be able to read the help from their website too, not sure.
Best of luck!