Toucans have large, heavy bills, which can make them top-heavy. Their tails help to counterbalance their bills and keep them from toppling over.
Balance
The tail also helps the toucan to balance when it is perched on a branch.
Support
The tail provides support when the toucan is climbing trees.
Communication
The tail is also used for communication. When the toucan is excited, it will fan its tail out in a display.