PRICES GO UP AT THE GATE
They need this to move forward the series. Asuna needs it as well read ch 16.5 if you want to know why :P
They will also adopt a child ai named Yui in the game, she will also be their virtual child for the rest of the series
What does that infamous doujishi have to do with anything?
You'll understand if you read it. Plus that doujinshi explains a part that the novel didn't describe, the end of chapter 16. And if you still didn't know it's made by the writer of SAO. Just saying this, this explains how much Asuna really longed to be with Kirito.
I read that.I don't see people need it to understand their relationship
People view things differently. Don't waste your time trying to change theirs.