Though the Spencer style jacket began as a military garment, a waist length, double breasted coat. It was named for the 2nd Earl Spencer, George, who supposedly had a coat altered after its tails burned somewhat in the coals from a fire. It became a hugely popular ladies fashion that held sway for about 20 years.
During it's short primacy it had endless variations from dead plain, to lavishly decorated. As a practical garment it had little real value, since it covered so little of the body. But part of what it did do was allow a lady to be suitably attired for town with minimum additional clothing. She could grab her Spencer and hat, and be ready to go, where other options required greater time and effort.
There was another thing that made the Spencer an attractive accessory item. It was small enough that it could be made up at home quite easily, and required little material to create. Also, it was a perfect small canvas onto which a skilled woman could display her sewing artistry. It's size also meant a woman could use up small bits of things she had about, to embellish it.
Here are a few examples of this altered Attire word. It was charming, illogical, (as so many garments are), but in it's day, indispensable.