Australia's reason for not going 100% masks straight away was that there was little to no community spread and it would be a waste to use already low stocks of masks. My medical friends wrre very worried by low stocks on masks and faceshields early into this pandemic.
WHO initially had data to say that social distancing and good hygiene was enough to curb the spread but because this is a new type of coronavirus the data was limited on how it spread through droplets. We're now at a point where we're putting all the pieces together and we can learn from all the different approaches taken by different countries. A uncovered sneeze can carry coronavirus up to 8 metres. Two people wearing masks can stop a sneeze from spreading by a considerable margin.
The advice initially was that if you had corona virus that wearing a mask would prevent you breathing and coughing on people. However if you were negative the masks were pointless as they acted as a way of transmitting because as you exhaled the mask became moist and enabling the virus to attach to the mask. Hence the advice to not wear them unless you were positive.