Giants of all types always drop big bones which can be useful for Prayer training, though regular Giants are a more efficient source of big bones. This is due to Moss Giants spawns being relatively spread out.
These are the only monster in the game to drop Spinach rolls. When Classic was just beginning, there was a popular scam where players overstated the rarity of the item to uninformed players. This allowed them to exploit the knowledgeable player for whatever means that they could. There is only one other way to acquire this item, Moss Giants are the fastest way, now with the crystal chest being broken as of the time of this writing.