1 LIGHT Tuner + 2 or more non-Tuner DARK monsters Once per turn, you can send 1 LIGHT monster from your hand to the Graveyard to select 1 Level 5 or higher DARK monster in your Graveyard. Special Summon that monster. It cannot be used as a Synchro Material Monster.
Use a level-6 DARK monster that is easily special summoned like The Fiend Megacyber or Alien Overlord. Then summon Kinka-Byo and use it's effect to revive a level-1 LIGHT Tuner like Hanewata. Use all of them to Synchro into Chaos Goddess.
There are lots of decks centered around DARK monsters which could make use of Chaos Goddess' effect. Examples of DARK decks that contain many high-leveled monsters are Vampire, Yubel, Archfiend, and Gravekeeper.
Keep in mind that this card was created to specifically support Chaos themed decks (which is mixing DARK and LIGHT monsters) so it's best advised not to just splash LIGHT Tuner monsters and some DARK monsters in your deck and expect to pull her effect off, she needs her own deck to truly flourish.
Konami's logic: when it comes to free cards, they tend to make it's rarity higher than it should be so players will feel a sense of "achievement" when they own it even though 90% of free/ event cards are trash.