It has a small function. A random gamma ray decides to absorb into DNA? Well, 85% chance it won't matter.

My understanding is that on average a human will make malignant cell about once a year, but the other anti-cancer systems of the body deal with it. People that develop cancer had one of those cells escape the system. Without introns this would be a much more frequent event.

Similar idea: at work we were sintering metal powders together in a vacuum chamber, but had oxygen diffuse into the metal about twice as much as our limit to keep it strong. So the lead researcher took titanium powder that weighed a little more than twice what our work part weighed and put it in the chamber, and the oxygen level dropped to about half in our parts. After that he started making three parts at a time to keep oxygen levels down.

It's not a reason for developing this way, but introns are great for familial testing. They don't need to be preserved so they're changing all the time. If we didn't have introns, familial testing would have to be done by looking at several DNA or protein types. Blood type, what D2 receptor phenotype. What MTHFR phenotype, etc.

Sorry for long post but I love this topic: in all primates non-coding DNA, there's the gene that makes vitamin C. Most mammals make their own, except primates and Guinea pigs. In primates, the vitamin C gene is broken at the same location. So, the chances of multiple species of primates developing this mutation at the same place is very low. They'd all have to develop this mutation at the same place and thrive over the other members of the species, and become the dominant phenotype, and all offspring consume enough vitamin C in their diet to thrive also. Every single primate!

Instead, it's much more plausible that one primate or primate ancestor develop this mutation in an area where the food sources had plenty of vitamin C, and a population reduction caused this ancestor primate to be the sole ancestor of the remaining primates, which then evolved to become monkeys, apes, and humans.

The Guinea pig vitamin C mutation that causes it to not work? It's in a different location than the primate mutation.

Edit: apparently some primates can make vitamin C. But most can't, and the ones the can't, the gene is broken at the same place.