Is it sort of a decadent vestigial poncho? I can’t see a Victorian “Opera Man” cape being practically functional in any way.
Cape.
A cape gives warmth and coverage without binding. They are not closely fitted, and so someone who wears one is able to wear bulky clothing underneath.
I have several capes, which I wear instead of sweaters or coats. They’re much easier to get on and off, without stressing my arthritic joints, and I can easily drive in them. A coat or sweater used to bind my arms when I drove, even when I was quite thin.
Useful for horse riding, cheap and easy to make. Easy to take off, much easier than a soaking wet coat.
I always thought capes were meant to swirl dramatically about you as you turned to depart.
They are obviously important clothing for Evil Overlords.
Capes are probably the first clothing ever invented. Don’t have to mess with armholes, sleeves, tailoring. You have to be tall to wear a cape successfully, or else look like a mushroom, or a hobbit.
The practical functions of a cape have been addresses above. The sartorial function of a cape is to look cool.
Every snuggle up on the couch with a blanket? A cape is just a handy, portable version of that. Before effective indoor heating, I imagine capes were a great alternative to binding heavy clothing.
What about obviously purely decorative capes? It’s just showing off. It’s hard to picture now, but there were times and places where being able to afford a non-functional length of fabric put you pretty high up on the social ladder.
I have experience with this in Northern Cameroon. Ladies could usually expect to get one 6 ft length of cloth to make their clothes for the year. Rather than styling that into two functional outfits, they’d choose to use the entire length to create one outfit with puffy sleeves, layers of ruffles and, as the topper, a huge long non-functional length of cloth tied around the waist just to show that they could. I imagine there was a time when capes served the same purpose.
To strike fear in the hearts of criminals, a cowardly, superstitious lot.
Just to chip in here, there was certainly a time when cloth was a very expensive commodity indeed, and the purchase of such items was a lifetime decision, cloth was very much more hardwearing and intended to last decades, it was not thrown away either but was kept serviceable as much as possible.
Imagine a time when owning several outfits was a sign of wealth, but its true enough.
Modern cloth for the most part is far lighter, even denim today is lighter weight than that in the 197’0’s, and thats just in our lifetime.
The cost of cloth was very high right through most of the medieval period, and well into the reign of George I, organised factory production, which was subsequently mechanised led to the reduction in cost.
Just want to say good use of the word “sartorial.” Reminded me of studying for the GRE!
Most cloth in olden days was wool or linen. Very expensive, as noted, which is why you’re always reading about people dressed in ‘rags’. They wore whatever clothing they could get until it wore out. Travelling any distance by horse or horse drawn conveyance in cold weather was a long, tiring, cold ordeal, a heavy wool cape for warmth was extra valuable.
Thanks for brining back my childhood nightmares of trying to break in a new pair of Toughskins jeans 
Toughskins: your knees will wear out before these jeans do.
When you think about it, isn’t “to be awesome” a practical purpose?
Besides the ‘cool’ effect, a cape can be wrapped around the arm to use as a make-shift shield against a knife attack, to fight a bull, or to put over a mud puddle so the queen can walk across it and give you a chunk of a continent as a token of her appreciation. Magical capes offer a number of other capabilites, usually invisibility, but occasionally as a transporter device, or an extra-dimensional storage area.
No kidding; I remember when jeans were a canvas heavy enough to double as body armor.
For what it’s worth, FDR often wore a “boat cloak,” heavy wool ankle length cape. The is a photo of him and Churchill either on a war ship or at Yalta wearing one. I suppose it was handy for a man who had limited mobility and it concealed his legs as well.
What is even more impressive is the long green leather coat affected by German policemen.
Quoth Edna Mode: “NO CAPES!!!”