a) in my city I wear them because we have bad public transportation and people tend to be dangerously packed inside the buses (this issue recently has started to be alleviated), so to protect my self (and also show that things arent well inside) I wear the mask.
b) in other foreign cities I wear the mask in public transportation because:
1)I dont want to be sick on vacations
2)Different countries probably(?) have kinda different strains of viruses and such, so it may be more dangerous for me.