为什么Xcode / Swift坚持我需要为Delegate和Data Source方法使用override关键字？

The following code works absolutely fine if I use the override keyword as Xcode insists I do...but if I remove the override keyword (as seen below in the Data Source extension), I get build errors. I see people organizing their code in this exact same way all over the place, and yet they aren't using the override keyword...any help? I mean, obviously if I just add the override keyword, everything is fine, but I'd like to know "why?". Is this something new in Swift 1.2 and I'm just seeing a bunch of old code?The following code works absolutely fine if I u