That headline is marginally incorrect.
Nobody outside D.C. and parts of the Upper East Side cares about John Dean.
Bringing in the guy who has spent generations dining out on the Nixon White House stuff to testify was a publicity stunt aimed at a very specific public, not the general public which hasn't cared about John Dean since the invention of the Atari, but the D.C. political and media class.
That's whom House Democrats have been pandering to. Inviting John Dean was the equivalent of a literary society inviting an obscure Hungarian writer that nobody outside publishing circles cares about.
It goes without saying that Dean isn't relevant to any of the purported investigations. He's long been a talking head. He has nothing to say about Trump except the usual fulminating stuff. He's not a White House insider in this administration and he has no actual information to offer.
But if you're going to have a circus aimed at the D.C. political class, then of course you would invite John Dean.