What to use a 90 and when to two 45s in copper piping?
When to use a pair of 45° versus one 90°? Most in the construction trades work according to these guiding principles:

Use as few parts as possible, because handling more parts takes more time
Get the job done with whatever I have on the truck, because trips to the supply house and back to the job site waste time

With that philosophy in mind one would use a pair of 45° elbows only if using a single 90° is impossible given building constraints, or if a 90° isn't on hand.
As for the mangled fitting: that's not a coupler. It's much too long; there's no way a person could bend a coupler without kinking/crimping it shut. This looks like a 45° elbow that has been twisted about half a turn. Perhaps the faucet was installed upside down and somebody, working from the outside and assuming that faucets are always connected with a threaded adapter, decided to just twist it around to fix the orientation.