Because guys need a place to put their things, too… :-)
It is a well-known fact that most men in Tokyo are as fashionable as the ladies. That includes clothing, hairstyle and purse… I mean bag.
It may be odd in some other parts of the world, but that’s the norm here.
Function-wise, the biggest advantage of using “man purse” is the ease to handle when commuting on a packed train. It beats sling/messenger bag, backpack and briefcase hands down.
Me? No, I don’t use this kind of bag.
Why? Because my camera won’t fit in there.
*Saginuma Station, Kanagawa, Japan