Note the following points, with sources (The context is a bit off because I did this research in response to another question, but the points are still pretty much relevant here):
1. Regular old surgical masks (not N95 respirators), when worn by the public without training, had a strong protective effect for the people wearing them during the SARS-CoV-1 outbreak. They reduced the risk of infection by ~70%. (Source: https://wwwnc.cdc.gov/eid/article/10/2/03-0730_article)
2. Despite being very different in their ability to actually filter out fine particles, N95 respirators and surgical masks actually show no significant difference in their ability to prevent diagnosed respiratory infections including influenza, when worn by healthcare workers according to two large meta-studies. Though both had a protective effect over no mask. (Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4868605/) (2nd source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5779801/)
3. Improvised, homemade masks have only a marginally lower filtration efficiency than surgical masks (which we established are about as effective as an N95 respirator anyway). (Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440799/) (2nd source: https://www.researchgate.net/publication/258525804_Testing_t...)
4. There is evidence in the studies linked in item 2 that hand hygiene alone did not demonstrate much of a protective effect (for civilians caring for a sick family member) against transmission of influenza compared to use of any type of mask.
5. When infected people wear a mask, it helps reduce the amount of viral shedding which occurs, presumably making them less likely to infect others. (Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3591312/)