Well like most other colonialist european countries, they made a gain in terme of natural resources. Unless I'm mistaken, in Indochine, it was mostly about coal and anthracite (another variety of coal), but later on (from the end of the 19th century onwards), and this was the case for many countries/regions of South East Asia, natural rubber was one of the most important natural resources produced and exported, because of the emergence of cars and everything.

Also there are some "prestige" arguments that can be made to explain why some countries wanted colonies (and to have as many as possible), but since I don't know that much about colonialism, someone else should do it...

By the way, I don't really have an official source for what I'm saying, those are notes that I took in my South East Asia history class when I was doing my Bachelor's degree in History... So I hope it helps a little :p

The South of Vietnam is the food basket of the country.

Before rubber and coal, rice export was the main good exported to China and Japan, since the agricultural revolution hadn't taken place yet. And to this day those countries still buy a great part of their staple from other countries, especially China who has been importing a huge quantity of rice from Vietnam.