The answer can be found throughout history. Legislatures, and democracy itself, have come under attack from outside. Powerful interests have sought to intimidate Parliaments and parliamentarians. Parliamentary privilege is designed to ensure that legislators can do the job we expect of them.
MPs can bring up allegations in Parliament - which, without 'parliamentary privilege', could lead them to face the reality of legal action. Legal aid isn't available - and a powerful individual like Maxwell can (and in his case, frequently did) hire the most expensive lawyers. If the defendant has costs awarded against them, they can be financially ruined. In any case to defend an action they need to get good legal representation. Defamation law can be a great good - allowing an individual to protect themselves from malicious and damaging assertions. But it can also be used to suppress free speech.
Some sections of the press can do a great job in uncovering wrongdoing - but a newspaper's editors need to take into account the danger to their publication of being sued. That's what Maxwell knew - and counted on. Parliamentarians receive complaints from their constituents about the actions of powerful individuals and companies. Sometimes it is necessary to use parliamentary privilege to expose and hold to account. The use of privilege is taken seriously [the Committee of Privileges in 1986-87 stated "We should use our freedom of speech...with the greatest care, particularly if we impute any motives or dishonourable conduct to those outside the House who have no right of reply."- and a legislator who abused the privilege could find themselves in deep trouble with the legislative body itself.
In 1955, Kim Philby was named in the House of Commons as a Soviet spy. The academic author of a textbook on Constitutional Law, Hilaire Barnett commented - "...which surely would not have been made without the protection of privilege."
In its disputes with the Stuart Monarchs - and in response to James I's assertion that Parliament's rights were derived only from the 'grace and permission of our ancestors [former Monarchs] and us [James I using the 'royal we'] - the House of Commons asserted in its Protestation of 18th December 1621 that
"every member of the House of Parliament hath, and of right ought to have, freedom of speech to propound, treat, reason, and bring to conclusion the same"
Last week the Court of Appeal ruled on the issue last week - this report is from the BBC:
"John Bercow has said he is "absolutely delighted" that an attempt to sue a former Football Association boss for libel over comments he made to a parliamentary committee has failed. Lord Triesman made corruption allegations against some members of the sport's governing body, FIFA, in 2011. But the Court of Appeal ruled he could not be sued, as he had spoken while protected by "parliamentary privilege".
House of Commons Speaker Mr Bercow called this a "victory" for Parliament. Lord Triesman, a Labour peer and former minister, was part of the team involved in England's failed bid to host the World Cup in 2018. In 2011, when he appeared before MPs on the Commons Culture Committee, he made corruption allegations against some members of FIFA. Worwawi Makudi, former head of Thailand's football federation, attempted to sue Lord Triesman, former FA chairman, for defamation. Mr Makudi has denied any wrongdoing and Lord Triesman's allegations...
The Court of Appeal ruled last week that the comments to the committee had been covered by the legal protection of parliamentary privilege, established by the Bill of Rights in 1689, which bars MPs' and peers' proceedings from being "questioned in any court or place outside of parliament".
The chairman of the Culture Committee, Conservative MP John Whittingdale, said this was "a significant re-establishment of the rights of this House". Had the court decided otherwise, it could have had a "severe effect in terms of preventing us exposing truth", he added.
Mr Bercow, who wrote to the Court of Appeal last year expressing his views, said he had shared the "grave concern" of Mr Whittingdale and that he was "delighted... the court found as it did". He added: "That was a victory not just for Lord Triesman but for the precious principle of parliamentary privilege and a victory for Parliament itself." "
Erskine May, the handbook of Parliamentary procedure, defined privilege as being "...the sum of the peculiar rights enjoyed by each House collectively as a constituent part of the High Court of Parliament, and by Members of each House individually, without which they could not discharge their functions, and which exceed those possessed by other bodies or individuals. Thus, privilege, though part of the law of the land, is to a certain extent an exemption from the general law."