Facts tell: What's behind the change of tune on mask wearing?

We are moving towards an increasingly masked world. According to the UK's scientific academy, the Royal Society, masks are now compulsory in public spaces in over 120 countries. 

But this was not the case when the outbreak first started. In the early days of the pandemic, politicians and even public health authorities warned people against wearing masks. 

So what's behind the change of tune when it comes to mask wearing?