it is def necessary if u are applying for usa/aus/uk visa i am not sure about canada but it is just a notarary paper on which your sponsor in your case your dad will mention some details like he has such and such money in his account and property in pakistan and he will burden all your expenses from tuition fees to living expenses in Canada and u will not be a burden on the canadian govt while u r there. This notary papar will be notarized by a notary public. This is called Affidavit of support.
since u have got your medicals i dont think u should worry too much about it. Bank letter and bank statements would be enough if it is missing from the docs. However, it is advisable that this affidavit of support should be submitted alongwith other documents. It is like a halafnama/oath from your sponsor that he will not back off from funding your studies....