It's usually used when you are surprised to learn something. Other uses are to add how serious you are being about something. The last and most obscure usage is to explain the status of a couple. They are "For Realsies" if they are officially dating.
"Koalas aren't bears they're marsupials!"
Usage 2:(add sincerity)
"I have always dreamed of being a plumber"
"Are you serious?"
"Did you here Jack and Jill are for realsies now?"