The treasury screen is used to access the guild treasury which is where guild funds are kept. Any member of a guild can deposit money into this fund but only people who have been given a title that has treasury admin access can actually withdraw money from it.
The intended purpose of the treasury is to provide a way for members to help fund guild expenses such as new property or simply as a means to help out new players by having an easy access to a central fund of money.
Whenever a deposit or withdrawal is made a note is added to the note screen to indicate who did what and how much money was involved.