Okay first question, Why
That is a difficult question to answer, I myself like the characters and the way they interact. It's good clean fun and I enjoy it a lot more than a lot of other things that I could be doing. That and I find the images "Cute" (My god that is the first time I have used that word in a non satirical manner for 4 years) and funny so I prefer to use them, that and most of them are SFW unlike a lot of my reaction folder that is NSFW or Not Safe for Children.
Is this earth here or pony land?
Technically you could say that this is a series of tubes, but to be honest this is where most of the pony posting and discussion (if any) happens.