Tip: to get help and suggestions about that feature, simply type 'visual similarity' in the text entry box of the home screen.
As you'll discover, there are several goals which can be achieved by calcuating the degree of similarity of images.
What are you expecting particularly?
If the feature is important for you, I suggest to use the organizer option to 'repair' the catalog and tick the option to 'index' your library for visual similarity. That takes time but will be saved in the catalog folder for faster use afterwards.