Here's my hypothesis:

The sun is increasingly emitting something (radiation probably) that they feel could be extremely hazardous to our health and our lives. They are responding by "injecting" chemicals into the atmosphere that are meant to reflect or possibly absorb radiation. There are undoubtedly some ill effects with regard to our health caused by the spraying, but they have to do it--otherwise, the health care system comes crashing down as it's overwhelmed by cancer patients. The entire financial system--linked to the healthcare system in many obvious ways--follows soon after.

In other words, they'd rather have you somewhat less healthy than dying. Dying is expensive. I know that won't be popular on a conspiracy site where we all seem to think they are out to kill us, but if you remember that everything is a business model you'll see that the last thing the want is a bunch of terminally ill patients overloading the system. The costs would be astronomical.

And they cannot tell us--society would break down as people realize they're being radiated and that many will die.

Chem trails are just a business decision.