Visa interview: Will you return to your home country after your studies?

Why almost all students answer 'Yes'?

The host country is not looking to accept immigrants when allowing students to come pursue degrees on education visas. Each country has their own system, standards and procedures for allowing immigrants in the country, and if you appear like you're trying to skip that by entering as a student they will deny your student visa application.

You apply for a Study visa for a purpose of studying for a fixed term. This visa does not allow you to stay longer than the term of this study. After the end of your studies you should return to your country (even if your studies are completed or terminated before the expiration of your Study visa).

The question asks you to affirm that you are familiar with the rules and understand it. It does not really ask you about you life plans on a general scale (e.g. whether or not you want to apply for a work visa after your PhD or get married or whatnot), and these plans can always change. The question simply aims to establish that you understand that Study Visa has a limited duration, purpose and scope. That's why most people answer "yes" to this question.

If you decide to overstay your Visa or remain in the country when your studies are completed or terminated, it may be difficult for the government to remove you for the overstay (especially if your visa is still valid - it need to be revoked, you need to be informed, etc). But it it very easy to remove you for lying in your visa application - and the fact that you remain in the country despite staying the opposite in your application can be interpreted as such. This is (in my opinion) the real purpose of the question.

IANAL.