I actually did find it helpful. Because I still didn't really see what he liked about her after the series ended. I kept having the feeling all his sweet acts were just to cheer her up. But I came to realise he's just an extremely bashful character who doesn't know how to express himself. How he actually really likes her and admires her devotion. (Such idiotic devotion btw, I could never be like that, she practically lives for him...)
Though it was a bit too cheesy for me at times... xd Surely it's sweet, but sometimes the sweetness makes me cringe! xd And damn, he kept 'thinking' stuff like "I, the great Baek Seung Jo", which I found really annoying. I'm adoring KHJ's smile though... ehheheh...
They help the viewers to understand/know what Baek Seung Jo thinks in each one of the YouTube Special episodes.
But I agree, they should have made longer episodes instead of creating a diary.
(Sorry for my not-perfect English)