A place to ask questions, stupid or not.
It's trying to simulate tone over text. For situations like your example, the implied question is something like "why do you not know this?" Or "what are you talking about?"
Assume the person is giving you a quizzical facial expression on the other side of the screen, like they're baffled by what you've said.
At least from the example you gave, it is a question more than a statement. They're not correcting you so much as questioning your answer and offering what they think it really is. Basically, skipping the filler in asking "Are you sure its purple, not blue?"
I see your logic. I still wonder how this became popular though. Is it just one of those things people have been doing for a long time that I didn't notice, and then one day I noticed it and I started seeing it everywhere?
I think its always been around. It might be more common to see it without further qualification (IE, "It's blue, isn't it?"), possibly because of the popularity of short-form content, but I don't think its a new thing.
I think it's just typical evolution of language, which happens pretty rapidly, since each generation likes to distance themselves a bit from their parent's generation. It saves a bit of typing, and since typing is such a common activity, it makes sense, just like using 'u' instead of 'you'.
I guess I'm getting old. I do turn 30 later this year. :p