Confederation was considered a good idea because, once untied, Canada would become more strong and independent. This would not only improve Canada's reputation, but it would save them from being annexed by the United States as well.
Joining together as one would also provide a more stable source of income, and allow colonies that were not totally on their feet, to get back on track because they have the support of the whole Country.
(I am sorry if this is not a good answer... I am only a grade 8 student and I just learned this, so its a guess!)