From the CDC website: "If you are NOT sick: You do not need to wear a facemask unless you are caring for someone who is sick (and they are not able to wear a facemask). Facemasks may be in short supply and they should be saved for caregivers."
The study linked in GP seems to directly refute this.
"Overall protection factors calculated per type of mask were stable over time, and did not change statistically significant with prolonged wearing."
The quote from the CDC makes no claims about efficacy, only that supply is limited and healthcare workers should be given priority.
There is aplenty of reasons why healthy individuals should wear the masks, including:
1. They could be sick, but don't know it
2. Sick people avoid masks if healthy people don't have them
3. It could help them avoid touching their faces