**If you play a common deck or a deck with a common theme it's easier for your opponent make correct decisions.**

Say I've played against your "Goblin deck" a lot of times. I know what are your most dangerous threats, what creatures I want to kill, what spells I want to counter, how long I should wait before I play my sweeper, what your kill range is, and so on. 

So say you bring a deck I've never seen before, and play a bunch of cards I've never played against, I have a harder time to make optimal decisions against you. Does he win by milling or damage? Is he searching for a combo? What's his kill range? Do I want to counter this spell or should I save the counter for his next one? Can I risk playing a creature here?

To summarize, **playing an unknown deck/playstyle/wincon will give you an information advantage** (unless your opponent does too) to base you decisions on.