1. Protect others
Obvious reason is protection yourself from others just in any other cases. But, primary reason of wearing mask started to protect others. Remember, Japanese way of thinking is outside-in. Think others first and then think him/herself. In case you catch cold, i don't want to spread germ through coughing and sneezing. Doing so will be harming others. Coupled with high sense of hygiene, it is basic manner to keep you in the social context.
2. Protection from others and bad stuff such as pollen, pollution, dust
Same is true. But, interesting fact is about allergy to pollen. The allergy patient in Japan from pollen is dramatically increasing since its first report around 1960s, and it reached to nearly 1/4 of total Japanese are suffering from pollen. So, why did this happen so much? Interesting fact is that Japan has planted trees at massive scale after world war. Yet, there is less need for cutting trees as there are cheap importantion from Canada and South Eastern Asian countries. This led to more trees and now after 50~60 years after, this is peak season for pollen emanation
3. Protection from No Make Up
Many of Japanese women are suffering from lack of time, and busy as they join to the workforces. Therefore, no time to do many layers of make-up they used to do and known for. To cover this up, convenient way of wearing facial mask.
4. Hiding herself
It's purpose is evolving into hiding tool of herself. She and he doensn't want to show herself and protect her identity. She fears how she is being perceived, this means that she is vulnerable. When feel vulnerable, this is protection mechanism. Of course, the celebrity does it to hide their identify in the crowd so they can also live their personal life. But, in Japan, we see this happens across everyone not only for celebrity
5. Expressing herself
Nowadays, face mask is becoming one of the fashion item. Many mask claims not only functionally superior but also makes you look beautiful by forming a certain shape such as making face look smaller. In addition to traditionally just white coloured face mask for functionality, it has many decoration to be fashion item complementing overall look.