When I hear a pilot tell us to return to our seats and put on our seat belts I imagine a major turbulence is headed our way and they want us to strap in so we don't go flying around the cabin.
This isn't necessarily what the pilot is thinking, usually they just use that buckled strap of fabric as a crowd control device, keeping us seated and the aisles clear in case the s#%t hits the turbine.
Because as we all know, and this Extra Fabulous Comics comic shows, that strap attached to our seats isn't going to do a damn bit of good if the plane crashes- but it'll make the clean up crew's job easier!
-Via Geeks Are Sexy