TheBooBooKitty says "The mace became a symbol of power and status during the middle ages. Higher ranking Army Officers and Nobles would often carry highly decorated maces that had been produced especially for them. The owner of a mace was usually recognized as a person of power and influence."
This still very much applies in the case of the ceremonial Maces used in the British Houses of Parliament.
Essentially, the Mace in the House of Commons is a sign of the Queen's authority. It is a silver gilt ornamental club of about five feet in length, dating from the reign of Charles II. It is carried in as part of the Speaker's procession by the Serjeant at Arms before each day's sitting and placed on the table of the House, except when the House is "in committee" when it rests on supports below. It is then carried out when the Speaker leaves.
Michael Heseltine during a debate in 1976 once famously picked up the Mace and waved it over his head. He was restrained by another MP and replaced it, and the Speaker suspended sitting for the day. He then apologised the next day.
The House of Lords has two maces, one dating from the time of Charles II and another from the reign of William III. One of the maces lies on the woolsack when the House is sitting and is the symbol of the royal authority under which the House meets. The other is carried with the Lord Chancellor (who is ex officio the Speaker of the House of Lords) when he is performing his other official duties.
During the State Opening of Parliament, the mace in the House of Lords is removed. With the Queen sitting on the throne at the end of the House, there is clearly no need to have the mace there to represent her authority!