Originally Posted by
Soukyuu
Here's the elaborate version on that:
Garlean Empire's motivation is "Primals are killing the planet, so we have to stop them. Unite under our banner to bring on an era of peace! If you go against us, this means you are aiding the beast tribes to destroy the planet and are our enemy." Thus, the conquest is only secondary in the sense that they don't want the land or the riches per se, but are focused on getting as much power as possible to get rid off the primals and their beast tribes.
Basically, we're both fighting for the same cause, but we don't want to unite with the Empire and lose our independence. Yoshida-san said he wants to keep this ambivalent relationship a little longer. Maybe it's a hint to a future alliance with the Empire?
Yoshida-san also said that of course, due to human nature, there are cases of aggression purely out of greed, too, so it's not like they're some kind of saints.