It is mainly used during the Monkey Madness quest where players need it to pretend they are the envoy of Karamja monkeys. When wielding the greegree, the Spellbook is disabled, but tablets can be used.
A Hard Ardougne Task requires the player to hold a monkey greegree and talk to a monkey minder, who will put you in the monkey cage. To get out, just unequip it, and talk to the monkey minder again.
The different greegrees can be combined by using one on another. This adds both of the greegrees to a list that can be accessed via the Swap option on any of the greegrees that the player owns, and in the process deletes the greegree that was used on the other. The greegree can then be turned into any of the greegrees by selecting it in the list. The greegree cannot be swapped into another while it is equipped, but it is possible to equip another to change forms.