It is, as Downunder says. The main reason was coolness. It gave the look of an extra layer without the bulk. The design originally comes from military mess dress uniforms. Messes were traditionally, cold and draughty, or hot and stuffy. Wearing this, if you got cold, you stood nearer to the fire. If you were to hot, at least you didn't have the extra layer!

I have three currently, but am looking to possibly convert another two.

Regards

Chas