Well for one thing there are a thousand times more resources spread throughout the solar system then there are on our planet. Entire oceans of hydrocarbons. Asteroids that are 30% platinum. Would seem kind of silly if we ended up killing one another for access to resources when they are vastly more available beyond the gravity well of the earth.

Learning to exploit those resources leads into the idea of moving our heavy industry off planet. Would sure be nice to turn earth into one big nature preserve without losing our capacity to produce the goods we want.

Thirdly you don't just start building a generational ship to get to explore the nearby habitable exoplanets when we find them. We need to have an entire industry devoted to mining, refining, and construction in space if we want to be able to build such a thing.