Congrats on your interview! I agree with the other experts here.
On why consulting you should be able to give 3-4 reasons that justify why you want to enter the industry. Typical answers would be:
- Getting exposure to a wide variety of industries, functions, and problems
- Learning the consulting tool kit (e.g. hypothesis driven problem solving approach, ability to communicate well with clients, analytical capabilities, etc.)
- You enjoy working together with other people in small teams trying to solve though business problems
Ideally, you can tie it to something about your character to make the answer more convincing. Could for example be, "I am naturally very intellectually curious and like to learn about different domains, so seeing many different industries and problems suits me perfectly as a person"
On why Bain Brussels you should have a few answers related to Bain as a firm and then ideally something more specific as to why you want to join Bain in Brussels. Your need to justify your application to a particular office also depends to a great on your personal background (e.g. are you Belgian). If not, this becomes a more important topic for the interviewer to test with you.
There should ideally be a strong personal connection to Belgium / Brussels for there to be a coherent story to tell the interviewer.
It is also worthwhile to understand the client base and the types of cases in that office as a justification for your application. In the case of the Brussels office they have a strong expertise in the Beer industry for example which could be worth discussing, if that is something that interests you.