I don't exactly agree with Underway here.
First thing, in the US, the party's are electoral machines primarily, and political philosophy straight-jackets a far second. Their purpose is to get you on the bulletin and then, raise funds for the campaign to get you elected. That's why the party don't have a platform applicable to all "per se".
In Canada, we let the party's bureaucrats take over the process of naming the candidates, and we let the party leaders take over the parties from the base (as counter intuitively as it may seem) when we adopted the American practice of leadership conventions that took away from the elected MP the power to appoint or remove the party leader.
In a true British parliamentarian system, it is not the leader of the party with the most seat (though it usually ends up being the one) who becomes PM, it is the person - any person - asked to put a government together by the Monarch and who happens to be able to obtain, and retain, the support of the majority of elected MPs. That is why the caucus of a given party used to be the one appointing their party leader, a situation where it is for the leader to obtain the support of his/her caucus and maintain it at all time. In such system, the leader needs to get that support from the real elected representatives: the MPs.
By having the appointment of leaders, and their removal, moved to the members of the party at large, either by conventions or vote at large in the party, it reverses the whole system and now, it is the future MP's who now owe their standing to the leader who is imposed on them, with the leader holding power over their heads instead of the other way around.
Our current system may give the illusion of democracy since the "PM" is "elected" by larger number of people (the party faithfuls) than just his caucus, but in practice, it does the reverse and basically puts in place a temporary dictatorship of four to five years by someone (the PM) that, in the end, few people had a hand in selecting. At least, even if the number was smaller when the leaders were selected by their caucus, the MP's made their choice as an expression of the will of their (the MP's) electorate - and in the end actually represented a much larger portion of the population.