Well, Whadya Know

Why do we say that something important is not to be sneezed at?

Sneezing was once thought to clear the mind. That was one reason why wealthy people used snuff, inducing a sneeze by sniffing the stuff. It got to the point where the idle rich in their idle conversations would frequently force a sneeze to show they were bored. By reverse logic, something important, of substance, became something that was not to be sneezed at.
[source: Why You Say It by Webb Garrison]

Why do they turn off the cabin lights on planes before takeoff?

All the better for you to see the sights outside, folks. There's nothing technical about it, and it's that simple. At night, especially, the lights are quite pretty, particularly once you're airborne and you can see them sparkle all over the city. So why don't they do blackouts for landings? Because people then are more concerned with getting their stuff together for departure.
[source: What Are Hyenas Laughing At, Anyway? By David Feldman]

Why do we say that people who get hung up on minor arguments are "quibbling?"

You'll never guess what profession is the source of this word. Yes, it's our friends the lawyers. So, how do we get from them to the word "quibble?" It's from the Latin, "quibis," a form of the word "qui," or "who." Quibis is the equivalent of "party of the first part." So to quibble, in other words, is to talk like, and therefore to act like he or she whom you should usually try to avoid at all costs.
[source: A Browser's Dictionary by John Ciardi]