Hi,
Basically, what they are asking is that even though you are going for studies, what is the purpose of your wife's travel?
Depending on your situation, you could just write that she wishes to accompany you and she will only stay there as long as you're studying. There is an open work permit system for spouse of student studying in Canada. You may apply for an open work permit for her based on that. In that case, you will mention in the purpose of travel that she would find work based on her qualifications and such and accompany you in Canada for the duration of your studies (you can attach her education docs if that's the case). Remember that if you do apply for open work permit, your processing may take longer because then they will process both your applications. You can even have the purpose of travel document written from her side (with her signature)
Best of luck!