Raising the stakes. The highest stake in any movie is the life of the protag.

As Blake Snyder put it, the 'whiff of death' adds tension and raises the stakes -- and also grounds the story in a very real emotion for everybody: we've all been touched by death in some way, and are aware of its presence somewhere in our future.

But more so, I think it's along the lines the hero's journey as a metaphor for every stage of life, for every major change we experience or put ourselves through: we gather new knowledge, new skills, gain a new perspective on life and the world -- and the old Us "dies" and we are reborn as someone new.
If we are living life write, that happens often: we grow and renew.

The only alternatives are stagnation or death.

Man, typos in comment post-posting are embarrassing.
"If we are living life Right..."

There's also the matter of unfinished business. No matter how bleak things are for our protagonist, there's always a chance he could come back and set things right. Even broke, paralyzed, and deep in the heart of the Amazon, he might still find a way to solve the problem.

Death is the ultimate win for the bad guy.

Take your Protag to Hell and leave him there to claw his way out. It will give him so much more skin to wear. Simple as that.

If you have written a really good script and the director has bought into your vision, then the audience should feel as if they ARE the protagonist. They should have become so absorbed in the story, so linked to the protagonist, that they have inserted themselves into the story and by default, if the protagonist is in jeopardy -- they are in jeopardy.

One man’s opinion -- mileage may vary -- batteries not included.

Because "primal" sells, primal stuff grabs us, and the fight to stay alive is as primal as you can get.

As Blake Snyder wrote, "why is wrestling the longest running program? Cause it's primal. Two guys trying to kill each other." The audience knows it's fake and that it's just a show, and yet they keep watching, eager to see what happens next. It hits us on a gut level.

I think it has a lot more to do with change.

Stories are a metaphor for life. When one facet of life is over, death of the old must occur to make way for the new.

In film, that usually means that toward the end of Act Two the protag will face something that dramatically alters everything before it. He has to either change or be forever stuck.

Act Three is usually the proof whether or not that change has been for the better or worse (or if the hero will even stick with it).

Simple really.

ALL drama is about death. Even rom-coms. And the fear of death is what motivates our characters.

Rom-com: "If I can't be united with my true love, my life will be over."

Action: "If I don't defeat the antagonist, I will die."

Comedy: "If I don't get back on the air as San Diego's leading anchorman (and get the woman of my dreams), my life will be over."

Coming-of-age: "If I don't learn how to grow up, I'll never achieve my dreams, and my life will be over."