Kids, well girls, usually love fairies. It's a classic part of childhood for them. So fairies are a bedroom decor theme that's popular and loved.
Throw pillows make a room soft and comfy. Fairy throw pillows make a nice addition to a fairy themed bedroom because they give some extra color and brightness. They're an easy way to create a themed room too if you don't want to spend too much money.
Now the only issue you might have adding some fairy throw pillows to your daughter's room is finding some good ones. They can be hard to find but I have been able to track down some that aren't too grown up for a child's room.