As I'm typing the title, the system volunteers a list of posts inspired by the words "frequent" and "tab" but despite this, it's evident by the relatively low number of upvotes that the tab does not generate much attention or interest, which might explain why it is hidden under more on the "All Questions" page.
If I'm not mistaken, the frequent tab was created in 2011 as a means to record the increasing load of duplicate questions.
When many questions are closed as a duplicate of one specific question, that question is a "Frequent" question - that is, it has been asked, in different guises, frequently. In addition, if the question is linked to from another question, it is one of the "Frequently" linked to questions.
In essence the sort is for questions that have been linked to often within the site.
If you hover over the tab, the tooltip is "questions with the most links" - that is, with many questions linking to it. source
Who actually visits the frequent tab? – New Stack Overflow users? I doubt it, as I mentioned earlier the tab is hidden. Experienced and hi-rep users across the network? Maybe. Does anyone go beyond the second page looking for that canonical question to close the new one?
How useful is the frequent tab to communities? I feel that the side panel that shows related questions is more helpful for users searching for answers, and for those users who smell a duplicate a mile off. What am I missing?