When was the last siege tower used?
They still used them during the Napoleonic period.
Obviously Its A Scaling Tower With A Battering Arm Attached!!! _ _ _ Phil: Ummm, "obviously" the last guy didn't know (See what I did there?)! What I think you are looking for is a siege tower. It was used specifically in the First Crusade and was essentially a tower with wheels that could reach the extent of Antioch's walls.
From about 300 B.C until about the end of the medieval era when cannons became so effective that siege towers were obsolete. If you're asking the question "In what situation was the siege tower used" then the answer is when the walls were too thick to shoot down with heavy artillery or when the army's budget was so low that thy couldn't afford cannon.
Siege towers were of benefit in breaching enemy defenses/fortifications . For those within the siege engine itself they were in danger of having boiling oil scalding them and if the tower were set alight then those trapped could be burned alive or subject to missiles as they attempted to exit and flee from the tower itself .