I've both of them and both are very usefull.
One is a classic parallel guide that works very well, the other is usefull for cutting shadow gaps - and it enhances the dust extraxtion when used in combination with a dust extractor. It closes the big opening on the right side of the saw so no dust can come out there and the amount of air the dust extractor draws through this opening is reduced too - so more dust goes to the dust extractor.
Regards
Markus