"All types of masks reduced aerosol exposure, relatively stable over time, unaffected by duration of wear or type of activity, but with a high degree of individual variation. Personal respirators were more efficient than surgical masks, which were more efficient than home-made masks. Regardless of mask type, children were less well protected. Outward protection (mask wearing by a mechanical head) was less effective than inward protection (mask wearing by healthy volunteers)."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440799/
"A more specialized mask, known as an N95 respirator, can protect against the new coronavirus, also called SARS-CoV-2."
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."
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
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.
That said, it’s known that you can be infected without showing symptoms. So if you only start wearing one when you show symptoms, it might be to late already.