Dhanasekaran,
parallel loops can be used when the result in one iteration is independent of that of iterations before. Imagine you want to enlighten an image. You can do this for each point of the picture sequentially, but this would be done by a single thread on a single core of a single CPU. Instead you could utilize several threads, even cores and/or CPUs when using parallel loops. This way the points of the image are divided into subgroups and each subgroup is processed independent of the others. Results is a quicker overall-processing and a better utilization of your hardware.
See also here:
https://www.ni.com/docs/en-US/bundle/labview/page/configuring-parallel-for-loop-iterations-to-tweak-...
