1) An active space program creates numerous jobs, excellent ones that pay well and provide benefits. Most of these jobs connect directly to NASA facilities, making them difficult or impossible to outsource overseas. Because manned space exploration is more complex than robotic missions, it creates even more jobs. Right now, America needs all the high-quality homelocked jobs we can get.
2) Space exploration generates technological advances. Many of today's materials, computers, and other technological marvels emerged from the massive space efforts of the 1960s-70s. It's easy to forget how much we have already gained because now it is so commonplace. The more we do in space -- especially with human beings, who require more support than telescopes or rovers -- the more new discoveries we will make. New technology yields many benefits including improved safety and health, more efficient energy use, and new business opportunities.
3) Space offers a unique opportunity to develop solar energy technology. Above the Earth's atmosphere, the Sun's power is undiminished. Solar power can provide energy for equipment in space, but we can also use it down here. The faster we learn to take full advantage of solar energy and other renewable sources, the sooner we can end our dependence on fossil fuels. The more we try to work with solar energy, the more we will learn about how to gather and use it most efficiently -- and humans in space need plenty of energy. In this field, discoveries in space can translate directly into discoveries we can use on Earth.
4) Moving some humans into space can lower population pressure here on Earth. We're putting a huge burden on Earth's biosphere, and while emmigration isn't the only solution, it does help. Having somewhere else to go also lowers social tension by providing an outlet for people who want to leave, as the frontiers once did. For this to work, we need sizable colonies in orbit, on the Moon, or elsewhere.
5) Extraplanetary colonies also spread out humanity so we don't have all our eggs in one basket. At the rate we are damaging our environment and threatening each other with weapons, this is a vital step towards species survival. Numerous people involved or interested in the space program have cited this need.
6) With a thriving population of humans in space, we can get industry off the Earth. Much of the damage we do to the Earth comes from mining, manufacturing, and processing. Some of that could be done elsewhere, in an environment less vulnerable to harm. In particular, extraplanetary manufacture of equipment needed in space saves the effort of hauling it out of the gravity well. The Moon and the asteroids offer a variety of raw materials.
7) Space exploration demands the best of us. We have "civilized" the Earth to the point that many of us live in an environment that allows us to become soft and lazy. America was settled by explorers, adventurers, heroes ... people who believed in seeking out opportunities. We get bored and self-destructive without a challenge in front of us. Space doesn't allow that kind of coasting. It will kill you if it can. In space, you are alert and ready to face challenges, or you die. Humanity needs that rejuvenation to undo some of the damage we've done to ourselves in overpacifying the Earth.
8) Manned space exploration inspires people in ways that unmanned missions cannot match. Right now, people feel despondent; they need something to believe in and participate in that will make a real difference. They need the kind of hope that has both symbolic and practical aspects. People need opportunities to achieve great things, and space exploration is a tremendous adventure. Robots don't excite them much. Heroes do. The real power of space exploration is this: when we send people Out There, they are lifted by the hands and the hearts of everyone involved in the space program, from engineers to taxpayers ... and we fly with them.