During damage calculation, if your "Destiny HERO" monster battles and this card is in your GY: You can Special Summon this card, make that monster unable to be destroyed by that battle, and if you do, you take no battle damage from that battle. If Summoned this way, banish this card when it leaves the field. You can only use this effect of "Destiny HERO - Dreamer" once per turn.
Since the effect only works for Destiny Hero monsters, it's mostly going to be useful in a Destiny Hero only deck. Destiny Draw or D Cubed can help with sending it to the graveyard early, special summon it when your D Hero monster is about to get destroyed so you can get Destiny HERO - Plasma or Destiny HERO - Dogma out. Alternatively, you can use Magical Merchant to send it to the grave, which can be good if you have multiple copies of dreamers.
You can use Drilldark to special summon Dreamer from your hand. This can let you make some offensive plays if you also use cards like Enemy Controller to tribute it and take control of your opponent's monster. D-Fusion can also be good to catch your opponent off guard. Even when you only have 1 monster on the field, you can special summon Dreamer to protect one of your monster(s), then you can go into Destiny HERO - Dangerous or normal summon D Hero for Vision HERO Trinity.