This setting, when enabled, prevents the Search Appliance from fetching URLs during custom "Find Similar" searches.
The /similar.html URL function (here) can execute Find Similar searches with arbitrary URLs. The text of the URL is needed to formulate a Find Similar query. To find it, first the URL is looked up in the profile database. If it is not found, then only if Prevent Find Similar Fetch is N will the URL be fetched to find its text. By default, Prevent Find Similar Fetch is Y, so that users cannot cause the Search Appliance to perform arbitrary URL fetches.