At the time they were working on a number of products, but they truly had a mastery in understanding the creation and sharing process of people between 15-30. It showed within each product they created and it was ultimately 9gag that went bananas in terms of user adoption.
They also have a pretty neat mission that extends beyond a r/funny wrapper -- though with the myriad of clones and previously created similar sites there is some credit due to them for their rapid expansion.
I hope the raise will give them the resources to expand further as I have no doubt they are already doing quite well.
As for why YC -- probably to connect more fully in the Valley since they are (or intend to be) based in Hong Kong.
Pretty connected names in the investor list over at angel.co/9gag // Ben Ling, Chris Sacca, Kevin Rose etc..
Oh, and while I agree to some extent that sites like these marginally improve the internet, humour is a great remedy to many of our ails.