Walking down the street.

I think is for protection

It is for her protection

I think the man is trying to secure the woman from road users. It is all about safety provision.

I would say in modern society, or western, he does so to protect her from either traffic or debris that will come from the vehicular traffic on the road. Ie. Chivalry!

I think too from perpetrators and other negative and these days, excessively angry folks on that street.

I was told that the reason a man walks on the outside is to protect the lady for any harm or danger, whether it’s his sister, mother, his lady, or other female relative and friend.