Just an observation. Joe is selling books. And DVD courses. And selling gear these days, using YouTube and other video channels to demonstrate the power of his sponsor's brand of speedlights and cameras, as well as his sponsor's brand of grip gear, and light modifiers,and his sponsor's brand of triggers. I watched a few of his commercials the other day, 9- to 12 minute ones, promoted through PetalPixel or fStoppers, can't recall which. So, it's interesting that you as a young, serious photography enthusiast are asking a VERY interesting and important question which was not answered by his videos...because his videos are covert advertisements for $549 speedlights, and $100 triggers, and $150 Joe McNally-branded softboxes and so on,and so on.
Doing 'educational videos', giving workshops, and teaching for money is one of the new, big things that the biggest names in photography are doing as a way to make money. And yet...the fundamental question you ask, the critical questions about "WHY TTL?" were not really addressed....and again, we circle back around, to what is being shown, and who is paying the production costs for these videos. I watched those two of four videos, as Joe very deliberately used $1,500 or so worth of flash and grip and modifier gear to shoot SIMPLE headshots outdoors in a DEEPLY-shaded area, where there would be NO sunlight...in order to keep synch speed below the level of ambient. I watched as he used a modifier that would make his $549 Nikon TTL flash unit's built-in remote commander triggering system INEFFECTIVE, so he had to put on Pocket Wizards to trigger.
He uses a Nikon D4, which HAS NO built-in flash, so he used an SB-910 in the hotshoe, in order to command and trigger a SECOND SB-910 in his modifier. So, two 910's, that's $1,099.90.
It would be a shame if you bought a pair of Lumopro 180's at $199 a pop.
Midwest Photo Exchange LumoPro LP180 Quad-Sync Manual Flash
You were watching the videos of a guy who is sponsored by a number of companies that make expensive flashes, high-grade triggering systems that cannot compete with the MIC ones on price, and by a company that makes very nice light modifiers that also cannot compete with the MIC stuff sold on eBay. And so I find it interesting that the videos did not really tell you WHY you ought to buy TTL flash units and PocketWizards...but they did show how totally awesome that stuff is.