The main reason of the interview is due to the specific visa officer having doubts about whether the relationship/marriage is genuine. This is what I have read on here countless times. Also, If the officer thinks even in the slightest sense, that marriage was entered to aquire stautus in Canada, or that there are some discrepancies in the answers by the applicant, or something that turned up in a background check, or a lack of concrete evidence, various red flags, etc. these are all some other reasons, plus the ones mentioned by above poster.
In my opinion, the interview is your last chance to bring forward documents, more evidence, to change the visa officer's mind that the relationship is genuine and continuing and not performed for the purpose of gaining status in Canada.
Here is an excerpt from a case that was denied an appeal. Note there was an interview conducted before the appeal.
"In assessing the genuineness of a marriage, the Immigration Appeal Division usually considers a broad range of factors including, but not limited to, how the couple met and how the relationship evolved, the duration of the relationship and the amount of time spent together prior to the wedding, the nature of the engagement and/or wedding ceremony, the intent of the parties to the marriage, the evidence of ongoing contact and communication before and after the marriage, the spouses’ conduct after the wedding, the level of knowledge of each other’s relationship histories, the provision of financial support, the partners’ families’ knowledge of and involvement in the relationship, the knowledge of and contact with extended families of the parties, the level of knowledge about each other’s daily lives, and plans and arrangements for the future. The factors are not exhaustive and the weight assigned to them will vary according to the circumstances of each case."
http://www.canlii.org/en/ca/irb/doc/2006/2006canlii52350/2006canlii52350.html