As Marcus Brody says, "nothing else had come close." Roll up all the priceless jewels and secret documents that every movie character in history has tried to acquire, and you'll still only have a thimbleful of the cool that this bad boy has. And best of all, it has some serious religious and historical affiliations, which give us a lot more to chew on than its simple presence as a plot device.
It looks seriously cool, for starters: inlaid with gold and with kneeling angels on top of it. This is in keeping with its initial descriptions in the Old Testament, right down to the fact that you have to carry it around on poles instead of touching it yourself.
Mr. Spielberg, master of lighting, further augments that by lighting the Ark with very subtle golden light, making it appear to shine and glow even in a darkened room. He also emphasizes its holy qualities with a signature riff from composer John Williams: something that starts quiet and subtle when just hinting at the Arks powers, then goes full-bore double-barreled orchestral when the time comes for the smiting to start. It lends a singular and very powerful impression: This belongs to God, and while you can look, you do not get to touch. (It gets, um, messy if you do. Apparently Belloq hadn't brushed up on Leviticus chapter 10.)
On the most basic level, the Ark serves the same function as the jewels in a robbery film or the secret plans in a spy film. Everyone wants it and will do anything to get it: That's the only thing about it that really matters. It could be petunias or rowboats or a six-foot resin statue shaped like the letter "g." As long as the characters will cheerfully knife each other in a dark alley to get their hands on it, it will work. Alfred Hitchcock coined a term for it—"MacGuffin"—and you can see it in everything from The Maltese Falcon to Guardians of the Galaxy.
The Religious Symbol
The Ark is a little different from those other MacGuffins, however, due to that whole "they wrote about it in the Torah" thing. It shows up in Exodus and was built to hold the shattered pieces of the Ten Commandments. More importantly, it represents a link between God and the Israelites: a physical sign of his presence on Earth. Through it, God told them where to go and what to do. His presence hovered over the Ark during their travels through the desert. He even used it to clear snakes and scorpions out of their path with cosmic flames. Handy.
But it was much more than just a cosmic CB and nifty pest-control device. It was linked to the power of God and that meant it had to be handled very, very carefully. If you approached it without the proper reverence, or failed to follow the complicated rituals involved, then your holy temple promptly got turned into the world's largest microwave.
Raiders of the Lost Ark incorporates all of those motifs into its presentation of the Ark (including its unknown whereabouts) in an effort to impress its holiness upon us. That makes it more than just a prize to be won; it's something to be treated with reverence. The Nazis don't figure that out, and thus get burned to a crisp. Indy does, and he survives. In the process, the experience might just bring Indy a little closer to God—or at least make him less of a jerk than he used to be.