SOP is a really important part of you application and helps you a lot even though it is optional.
Keep in mind that I have not received a response from CIC after I have submitted my SOP, but I had gathered info. from what most people said.
1. Be straight forward, don't tangle words. Visa Officer doesn't have time so make sure you don't annoy him by getting off topic or anything. Say everything very straight forward.
2. Make sure your SOP is not too long. 1 page is good enough.
3. State why you chose your subject.
4. Tell the VO about your previous education and employment(if any).
5. Let them know why you chose Canada(mostly because it is highly ranked and meets education standards etc.) and why the university you selected in Particular.
6. State why you will return to your home country(provide evidence of assets like a house or something and relations with family).
7. Tell your intentions after graduating from University.
