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.
1. Having just returned from a great workout you proclaim to yourself, "That was long, rough, and I will probably be sore tomorrow but it was worth it!"
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!"