Until recently I wondered what that check box did, must admit I didn't read the info tip that carefully. Even now I can't envisage any usage for its original purpose for myself - sites like Baen, Goodreads etc are largely irrelevant to me, and I put any relevant external links elsewhere.
This is what's driven my interest in this.
There was a recent change where columns (built in and custom) that appeared in the Tag Browser became 'clickable' in Book Details - publisher, #mytags, #original publisher, tags, #status etc. The click initiates a simple database search like clicking an item in Tag Browser. But template custom columns did not offer this facility.
I use the links in Book Details, most of my Authors have links to an external folder, my Notes column has links to external folders and/or external sites. And since they were implemented I also use the database search links - especially for custom columns, which are not available in Similar Books.
BR