Those nipples.

I was extremely suspicious about the effectiveness of simple masks and still am. They do stop you from sneezing on other people, obviously.

The biggest effect seems to be psychological - once the majority of people agree to wear masks that means they have taken control of the situation and are willing to do something about it, be responsible.

They stop most of the droplets of spittle you, and everyone else, spews constantly. Sneezing sprays the most, but just speaking spreads them way more than the 6 ft suggested.
If every American had actually quarantined themselves and worn a mask in public, this pandemic would be all but over in America, just like it is in most other countries besides Brazil who followed our lead, not still rising fast and still in stage one.

The benefits are mostly in reverse. You wear a mask because it better protects others from you - you should always assume you're infected, if you're truly being safe.

If you can get a medical-grade mask, then it can have additional benefits, but that's not really the point here. Everyone can and should wear a mask, and then everyone has an improved chance of preventing the spread.

It's not a psychological thing. Every time I go to the store and see someone without a mask, they are a greater risk to me and my health. I don't know if they are infected, and quite possibly, neither do they. That's the problem with this disease, the incubation and spread can be done entirely without awareness of infection. Blind assumptions are absolutely awful.

You can catch this disease today, and spread it without knowing you're infected, completely without symptoms. That's why we quarantine, contact trace, and above all, always assume you're infected and therefore wear a mask to protect others.

Now as for whether a simple bandana mask is worth it or not, hell -- we can talk, but I'm going to push everyone to do something rather than nothing. If I could get a medical grade protections I'd do it, but the point is when you have to have a worldwide effort, if this helps 20-50% more than nothing, I'll take it. Everyone should. To the point of this video -- there's no reason not to, the science has spoken loud and clear on these points.