The amount of strands used to manufacture a particular wire determines how flexible it will be. For instance, a 12 AWG hook up wire with 19 strands is less flexible than a 12 AWG hook up wire with 65 strands. The key thing to remember is that a 12 AWG wire has a diameter of .081 inches not including insulation. Whether it's one single solid strand of copper or it's 65 very thin strands of copper the diameter of a 12 AWG wire will total to .081 “.
Think about a solid strand of 12 AWG bare copper and compare it to a single strand of 30 AWG wire. A 30 AWG wire has a diameter of .01 inches and it can wrap around your finger multiple times unlike a 12 AWG strand. Since it's much more, flexible engineers determined that they can use multiple 30 AWG strands and wind them together as one, in order to complete the diameter of .081 “or a 12 AWG wire. The total amount of 30 AWG strands will conduct the same amount of electricity as the solid 12 AWG strand.
THHN THWN electrical wire is used to connect power from your electrical box to your outlets and lights through your home. This type of hook up wire is typically run through conduit or snaked through walls so flexibility is definitely not what they are looking to have. Electricians, contractors and installers want a wire that will hold its form while being pushed through Conduit. Therefore, a THHN # 12 wire has 19 strands instead of 65 to make it less flexible and easier to install in electrical applications.
UL1015 hook up wire is tested and approved by Underwriters Laboratories as 600 volts and a temperature rating of 105ºC. This type of hook up wire is typically used to manufacture wire harnesses that are installed into electronic devices and appliances. As an assembler it will be easier to work with UL1015 12 AWG wire because it has 65 strands of 30 AWG copper. Also, once the wire harness is made it is also a bonus to have each of the wires very flexible so that it can be installed easier in the tight areas of electronic devices.
Speak to your wire and cable supplier to figure out what kind of stranding would work best for your application. Some call for a flexible wire and some call for something a bit more stiff but, extremely, it's up to the preference of the user.