A recent study by a team in Hong Kong, the rate at which the virus was transmitted through airborne particles or respiratory droplets was lowered by as much as 75% when masks were used.
It was led by an expert who helped discovered SARS virus in 2003.
Check the research setting and results: it does not only prevent the infected ones from spreading the virus, but also protect the healthy wearers (reduce the chance of contracting the virus, or severity)
news.sky.com/story/coronavirus-wearing-surgical-masks-can-reduce-covid-19-spread-by-75-study-claims-11990381
(it probably explains why Asian countries control COVID19 better)