I did it online several days ago, on behalf of myself and husband. I translated every question for him so he could answer for himself. But he couldn't for the life of him understand the sexual orientation questions. He just kept repeating "Mais, je suis HOMME!!!" ever more angrily, so I just tried to figure it all out myself for him and filled it in appropriately. (He's not LGBTphobic, just confused!)
I do see that the Census helps the government to assess the numbers and location of many types of people - ethnicity, sexual self-identity, languages spoken and so on, in order to provide facilities, cater for needs and so on.