Lets give an example...
Say THB has been collecting my packs since 2006.
He has a folder full of all the packs and updates from then until now.
THB has many old files, many new files, many files which later found out had errors. etc. etc.
Take a look at these two files for example...
they are both Monkey Island 2 but one of them has an older slave.
In my DAT I keep only the latest version of every game.
MonkeyIsland_v1.0.zip would not be included in the dat because it has been replaced with the newer v1.1 version.
without going through manually, how will THB know which of all of his thousands of files are out of date, which ones have errors etc. etc.
Well, luckily for THB, I've created a dat.
If THB loads up clrmamepro with my games dat, it'll look through all of his files in a directory and by referencing against the dat it knows which files are old and useless.
Such as MonkeyIsland2_v1.0.zip
clrmamepro can then be used to keep only the uptodate files, removing all of the redundant files so you are right up to date!