Unless every single person is wearing a face mask, it's not really gonna do much at all.
Doesn't protect you, but it does protect others. Hence, everyone needs to be wearing one.
I'm not saying it should/should not be mandatory - don't really have an opinion on it. Just saying it needs to be a wholesale approach if it is enforced. And not sure you can really track it in any way.
In reality, the evidence isn't really that strong, but even a small impact would hopefully reduce the r0 rate even further, when combined with all other measures.
It'll certainly have the most impact if everyone wears one, but even a high percentage of the outgoing public wearing one would have some impact on the r0 rate.
In the long term, I'd probably advocate everyone wearing a mask outside the home alongside washing hands, but there needs to be adequate planning for this.
Will there be enough stock for the population? Will people have to buy them and if so where from or will the government provide them?