Parliament represented a necessary and crucial step in the evolution of government and the process of reform from the feudal and monarchical stages of history so that democratic government was possible. Even though the early versions were still subject to the King or Queen's autocratic rule, future democracy would not have been possible without its initial stages of existence.
The Summons of Representatives really layed out the first rules of representation of the people in the governing process. As mentioned above Parliament is the model of many Democratic governments.
I can but concur with #2 and the points made about the Parliament in Britain offering a genuine alternative - a kind of middle way - between monarchy and autocracy. The development of this far more democratic system of government has had a massive impact on all democracies as it paved the way for a more representative system of governance. Not that it is without problems, but at the same time it represented a real improvement compared to the abuses that occurred under monarchies, for example in France.
I think Parliament is important in Western History because it was the beginning of the idea of a limited and democratic government. The development of Parliament and its power showed people that there was an alternative to monarchy and autocracy. This idea led, for example, to the American Revolution and the creation of the democratic system we now enjoy in the US.
Parliament's development helped point the way towards the system of government that is used in essentially every Western country today.