Face masks were a relatively obscure phenomenon before the COVID-19 event started in March 2020; mainly used by doctors during surgery, in medical settings and somewhat popular in East Asia, mostly against air pollution. No studies of long term or widespread use of face masks had been done, for the simple reason that it was never considered a medically sound idea.
Points of note
The idea caught on and has multiple aspects, some of which are not fully understood, and might imply careful planning as part of a broader operation.
For a start, the mask functions as a muzzle, physically and metaphorically gagging people.
The masks turn people into faceless entities, the population into an undistinguished mass, which conspicuously fits with how the rulers of the world sees them.
The policy confuses and disorients people, making clear thinking difficult.
The masks function as a constant reminder of a medical danger that would otherwise not be observed. This has led to widespread insanity, with some people not having taken off their masks for years on end.
The masks function as a reminder of the state's power over you.
The masks function as a self-policing device, where people can monitor each other and social pressure will make people bow to compliance.
The masks, which are supposed to be changed several times a day, hugely increase the rate of pneumonia and respiratory diseases.
a. This contributes to the wanted COVID-emergency.
b. Long-term weakening of people's respiratory systems might be a goal in itself.
Masks function as a blackmail device, where good behavior (taking vaccines, allowing children to take the vaccine) will be rewarded with a promised removal of forced masks.
A large proportion of masks contain graphene, a novel industrial material also present in the the PCR-tests and - allegedly - the RNA-vaccines and with insufficiently tested health effects.
a. The thin and lightweight, yet tough and intractable particles "are notoriously worrisome in terms of the detrimental effects they can have on our health, particularly when breathed in…”
b. In addition, researchers from the EU Graphene Flagship project discovered that "injecting graphene oxide into a specific region of the brain silences the neurons responsible for anxious behaviour," making Jon Rappoport ask; "Are millions of people walking around in a sedated dumbed-down haze, because they’re wearing masks?"
Studies on Safety and Efficiency
As Swiss Policy Research pointed out "for decades, studies have shown that face masks don’t work against respiratory virus epidemics. But with the onset of the coronavirus pandemic and increasing political pressure, suddenly studies appeared claiming the opposite. In reality, these studies were a mixture of confounded observational data, unrealistic modelling and lab results, and outright fraud. The most influential fraudulent study certainly was the WHO-mandated meta-study published in The Lancet"
A May 2020 meta-study on pandemic influenza published by the US CDC found that face masks had no effect, neither as personal protective equipment nor as a source control. 
A Danish randomized controlled trial with 6000 participants, published in the Annals of Internal Medicine in November 2020, found no statistically significant effect of high-quality medical face masks against SARS-CoV-2 infection in a community setting. 
A large randomized controlled trial with close to 8000 participants, published in October 2020 in PLOS One, found that face masks “did not seem to be effective against laboratory-confirmed viral respiratory infections nor against clinical respiratory infection.” 
A February 2021 review by the European CDC found no high-quality evidence supporting the effectiveness of non-medical and medical face masks in the community. Furthermore, the European CDC advised against the use of FFP2/N95 masks by the general public. 
A July 2020 review by the Oxford Centre for Evidence-Based Medicine found that there is no evidence for the effectiveness of face masks against virus infection or transmission. 
A November 2020 Cochrane review found that face masks did not reduce influenza-like illness (ILI) cases, neither in the general population nor in health care workers. 
An April 2020 review by two US professors in respiratory and infectious disease from the University of Illinois concluded that face masks have no effect in everyday life, neither as self-protection nor to protect third parties (so-called source control). 
An article in the New England Journal of Medicine from May 2020 came to the conclusion that face masks offer little to no protection in everyday life. 
A 2015 study in the British Medical Journal BMJ Open found that cloth masks were penetrated by 97% of particles and may increase infection risk by retaining moisture or repeated use. 
An August 2020 review by a German professor in virology, epidemiology and hygiene found that there is no evidence for the effectiveness of face masks and that the improper daily use of masks by the public may in fact lead to an increase in infections.