Think of it as aging. It's giving your fresh ejuice time to blend and settle into itself . Some people use sonic cleaners and/or heat to speed it up. Some put it in the closet for a few weeks. Flavor typically develops and harshness is typically reduced.
Plain and simple,it's accelerated aging to meld the flavors to the carrier. i.e. pg/vg.
there are many methods but only through research and experience will you learn wich types of flavors and wich method to use for various vg/pg ratios.