PPE is designed to mitigate a completely different level and type of risk. PPE is designed for medical professionals who by the very nature of their job must spend hours in physical contact with known sick patients. The protocols must be strict because failure means guaranteed infection.
I on the other hand have no obligation to physically touch random strangers, thank god, and don’t have to go interact with known sick people. Instead I’m trying to provide some protection for myself, and trying to work in aggregate to reduce the risk that asymptomatic carriers spread the disease while doing essential activities. “Marginal benefits” in this context is also known as “reducing R0”, which is absolutely a public health goal.
Now all of that is a good argument for saving N95 masks for medical workers and relying on surgical or cloth masks for the general public, especially in the light of PPE shortages. But that is an entirely different argument from “masks don’t work”.
What's the point of wearing the mask then?
If you're wearing a mask to protect against droplets then maybe you'd get marginal benefits. But if your concern is virus carried in the water vapour that people exhale, then an N95 isn't nearly enough to protect you.
90% what?
It makes no sense to claim "90% effective".
90% reduction in numbers of people infected? Of droplets inhaled? Of droplets exhaled?
Without the rest of the kit, and without careful adherence to correct donning and doffing, there are at best marginal benefits to mask wearing.
There's some concern that those marginal benefits are eliminated if people wearing masks go on to engage in risky behaviours. These behaviours include increased touching of their face (often without immediate handwashing afterwards), reductions in distance between people, increase in frequency of leaving the home, etc etc.