My sister, a practicing MD, says that cloth masks are worse than nothing because they act as disease reservoirs. N95 masks are single-use. People aren't throwing their homemade masks after every trip outdoors, so any contamination their masks "stopped" is just waiting for the next use to infect the wearer. (She cited a study she'd read, which I can't find, but this Wirecutter article [1] makes a similar argument in the first answer).

That said, that doesn't mean that masks aren't helping in Czechia--they're a potent reminder to everyone to keep their distance, and that their friends and neighbors have bought into the seriousness of the threat. That alone is worth something.

1. https://thewirecutter.com/blog/your-coronavirus-questions-an...