Regarding C-24, plain and simple: the current government (and allegedly most Canadians) wished for a law enabling them to renounce the citizenship of those who get it and then leave Canada. However, such a law would be unconstitutional since it violates the Canadians' right to travel and live wherever they want.
So, bill/law C-24 was the best possible solution from the government's perspective for the perceived problem of the so called "Canadians of convenience". Now if someone chooses to leave Canada after getting its citizenship, the government can argue that he lied in his application about his intention to settle in Canada and hence it has a legal basis to question his citizenship status, possibly leading to renunciation.
Whether the government can actually prove that the applicant didn't have a genuine intention to stay in Canada at the time of his application or not is a different issue. The important thing here is that it can, using this argument, initiate a legal action and drag him into a lengthy and expensive legal process just to prove the contrary, or in other words: turn his life into HELL. No one knows for sure how the government will enact this clause, but it can be used in a very mean way against individuals. Just imagine that, while outside Canada, you receive a notice of citizenship renunciation because you "lied" in your application. You won't be even able to return to Canada to defend yourself in a court of law! Naturalized Canadians will remain under a constant threat of falling in this miserable situation whenever they travel outside Canada for a long period (e.g. for working abroad). This is exactly what the government wants.
Another benefit here is that the government can portray itself in the eye of the public as the defender of the Canadians' interests against those who "abuse" the rights granted by the Charter. Harper can always tell the public: Look, we did our best to act tough on the "Canadians of convenience", but our hands are tied by the Charter and the court verdicts.
