You're right about a good seal being important, but even t-shirt jersey cotton is better than nothing. Even if you're only stopping the larger droplets. If you reduce your viral load at all, you have a better chance of not becoming infected. So the worst masks could save lives, but they're nowhere near being good enough for ppl in high risk situations like healthcare workers
https://www.wired.com/story/its-time-to-face-facts-america-masks-work/