Islam preaches inner and outer modesty, and as human beings, we feel more safe and secure when we are covered up. This is why all religions somehow promote covering up and not being exposed for onlookers to see.
If a non-Muslim wishes to dress modestly, then of course that will benefit her. If she wishes to dress like a nun, or as Muslims, then that is also her choice, and it would not be considered as being 'inappropriate'.
In the case of a Muslim woman, or non-Muslim, wanting to, for example, enter into a holy shrine, there is a requirement that she does wear an abayah/chador. It is not Islamically mandated, but just out of respect.
And Allah knows best.