Need Items are items used to satisfy the Needs of Monsters in Monster Life, and are also how Monsters gain experience in order to level. The lower the level of the Monster, the less time it takes before that monster requires another Need Item.
Need Items vary in price depending on how much experience they grant to the Monster that uses them. Less valuable Need Items can be purchased with Gold, and more valuable ones are purchased with Crystals. There may be some variation between how many Need Items a particular monster needs to gain a level. For instance, Pumpkinster seems to require much more experience than most Monsters do.