Twisted multiwire springs are used because they have more surface area, and are more flexible than a single wire spring of the same relative force capability.
Compare the flexibility of a 3/16" piece of cable to a rod of similar size in the same material, and you will see what I mean.
This increases their potential life span in some cases, and would be a definite plus in very low temperatures, where embrittlement can be an issue. It can also make them easier to install, and remove in cramped design spaces.
These types of springs can be found in most of the AK type weapons, several other Finnish, Swedish and Russian designs, and on many other designs used in industry.