It may well have been a sewing room, used for sewing :)

Many women used to (and often still do) sew and need a small space for their sewing machine, sewing items, etc.

The other options might have been nursery, lady's writing room, small bedroom for help...really there are many options. All I know for sure...my mom would love to have a sewing room!

I did a little bit of research on this as I have a tiny room (smaller than yours) on the second floor of my 1876 Second Empire style home. I've seen such rooms called sewing rooms, dressing rooms, ladies fainting rooms, nurseries, trunk rooms (for storing luggage), servant rooms, and just plain storage. What they were actually used for I don't know, but I do like the idea of having a fainting room, with a fainting couch, to which I might retire when I get the vapors :).

Just a thought -- do the windows pretty much take up the lengths of the walls? If so, it could be a sleeping porch, used when the weather was warm. They usually have windows on two side for ventilation and are found on the north side of the home.

Yes, the windows fill up two sides of wall. So I also thought of a sleeping porch--or, the MN version of a sleeping porch which is 4 season and enclosed!