1. When you do not feel like elaborating on anything. Especially when the aforementioned phrases do not link in any way whatsoever. You would end the thought with "etc." because well, you just realized...you're an idiot.
2. When you do not wish to elaborate further on a topic where it is understood what happened next.
1. I had a can of peaches and I leaned into the dryer and realized that it smelled familiar and so I...etc
A wonderful "catch-all" notation that basically "lets you off the hook" and allows you to do whatever you please if all other parties involved agree to it.
One should be very wary about signing/agreeing to something that includes ", etc." at the end of a line, since this basically gives the agreement's profferer "carte blanche" or "the sky's the limit" permission to have/do whatever he/she pleases --- if someone squawks, he can always run back to that wonderful little "etc." word in da agreement and say, "Hey, you guys implied that I could do anything I saw fit, this is just one of those!"