1. You are alone, talking to yourself, and realize no one else is around to say "that's what s/he said" but it's such a good one you can't pass it up.
2. You are trying to seriously flirt and the other person is trying to be respectful/appropriate/otherwise no fun.
3. You are making fun of your own whorish tendencies to cheer someone up.
After realizing no one is around to take advantage of this opportunity, you can't help but hesitantly say, "That's what I said...?"
2. Person A making appropriate conversation about something: "It's pretty hard but I like a challenge."
Person B in overly sexual tone: "That's what I said."
3. Someone in your group of close friends is in some real trouble and exclaims: "Fuck me!"
You: "That's what I said!"
Your friend: "Hahaha, that is a true story you ARE slutty...laughing makes me feel so much better about how screwed I am!"