Today they are just figureheads. But when they had actual power,
Actually, that’s not technically correct. The Queen has considerable power. Every Act of Parliament requires her assent to become law*. She has the power to dissolve Parliament as well.
Granted, no British monarch has exercised those powers in a looong time (no monarch has withheld assent since 1707) — but in theory they do have them.
The Wolf
* So much so that even His Majesty’s Abdication Act of 1936 (passed for Edward VIII) required his Assent to become law. Therefore the very last act that Edward did as king was to (in effect) assent to his own abdication.