What is parallel modulation?
Similar to relative key changes, parallel modulation is about changing between major and minor keys. So its main function is to change the overall mood of the song with a simple chord change - major keys generally being upbeat and "light", minor keys generally being ominous and "dark".
What is the purpose of parallel key?
In music, a major scale and a minor scale that have the same tonic are called parallel keys and are said to be in a parallel relationship. The parallel minor or tonic minor of a particular major key is the minor key based on the same tonic; similarly the parallel major has the same tonic as the minor key.