Why do airplanes have more than one engine?

I am now, more than ever, a big fan of redundancy on aircraft.

On November 22 my wife and kids and I flew for a family thanksgiving in Pennsylvania. Nonstop from San Francisco to Newark on a Continental 757.

Somewhere in the middle, at 35,000 feet, I heard a strange revving of the engines, but I did my best to ignore it. Flying for me is a constant process of rationalization to deal with the thinly masked undercurrent of fear. I'm not a big fan of flying.

But moments later, the LCD screens on the ceiling folded up. Everyone glanced around, confused, because this had interrupted the inflight movie, and it was not accompanied by an explanation.

Then the seatbelt light came on, and somebody said over the loudspeaker, "Fasten your seatbelts." Not, in itself, unusual, but it was barked more tersely than usual.

A flight attendant walked by my aisle and whispered, blank-faced, to another: "Were you listening?" The second shook her head, and they walked silently to back of the plane together. This caused some concern among those of us within earshot.

Finally, the captain came on: "Ladies and gentlemen, we've lost all of the oil in our left engine, forcing us to shut it off. We've been flying for five minutes on the remaining engine. We're forty miles from Denver, so we're being diverted there."

Now, I know perfectly well that aircraft can fly with some engines off. I know that it's something that pilots train for. But that doesn't make it a good thing. Luckily for me--"luckily" in a strange sense--my whole family was with me, so I tried to act brave and reassuring. If I had been on my own, I probably would have whimpered and passed out.
The crew proceeded with a safety procedure briefing, which is interesting to experience midflight. They spoke even more simply and patronizingly than usual, as if they were speaking to children, or possibly to two hundred very nervous adults. They even re-explained how one buckles a seatbelt, in case anybody had forgotten.
On our approach, we were flanked by fire trucks ("Fire Rescue and Recovery", or something similarly sinister) every thousand feet along the runway. We landed without incident, and as we came to a stop there was a round of applause. (Unusual to hear nowadays--when I flew in Europe as a kid, every flight ended with a round of applause. I guess we take things for granted. But not on this trip.)
I was relieved to hear that they weren't going to ask us to to get back on that plane ("We jiggled some cables and it seems to be working again..."), but then we heard that the only spare 757 was in Newark--our destination in the first place, and an airport then experiencing weather delays.
Six hours later, the replacement plane arrived. There's nothing like six mind-numbing hours in a random airport with fidgety kids to make you want to get back on a plane after an emergency landing.
So a seven hour travel day turned into fourteen hours, and by the end of it I was overtaken by a cold that must have been brewing before, and which was brought about by stress and exhaustion. Thus I spent Thanksgiving vacation sick and a week later I still can't shake the dang cough.
Still, I can't help but feel that things turned out better than they could have. And for once, aircraft engine redundancy was specifically mentioned as something to be thankful for at Thanksgiving dinner.