There are more motives to go and explore space than to stay on Earth; confronted with the high costs of space exploration, some people and institutions consider that it would be better to use that money in a different way, but they forget to take the benefits of exploring space into account.
There are many things to solve here on Earth. Like poverty, injustices, authoritarianism, corruption, contamination, wars, diseases, environmental and climate issues and a lot of other problems. Confronted with that, space exploration and its thousands of millions of euros in investment might seem preposterous but in fact, going out of our planet produces many direct and indirect solutions to all those woes and then some. So here is a brief list of the reasons to go on exploring the cosmos:
Exploration is part of human nature: If we reject it we would also be rejecting part of our essence and nothing good could come out of that.
The universe is vast, and it's there: It is far bigger than our own planet, so it makes a lot of sense to see what is out of our atmosphere. Are we going to stay contended with what we already know? Are we going to deprive ourselves of new opportunities?
Overcoming barriers make us improve as individuals and as a culture, and evolve as a species: Great challenges make humankind advance in leaps, and solving the issues related to space travel itself and then colonisation correspond to that magnitude; small challenges only make us advance in small steps. The science and technology derived in quantum leaps from great challenges produce knowledge; ideologies, financial gains and political issues may be helpful but they are not paradigmatic and only produce advances in small steps and are generally of a short life span. Thus there is more to gain with solving great challenges than small ones because despite that it is indeed necessary to do something with them too, the gains that they produce are applicable only in moments in time, while knowledge is forever.
Exploration contributes to scientific, technological and spiritual advancement, and that, in turn, improves our quality of life and helps eradicate poverty: Just think about the impact of survey satellites developed as part of the broader advance in aerospace technology: They allow us to predict weather and climate changes, find natural resources, improve crops, protect the environment and so on. The money invested in various scientific and technological endeavours, including space exploration and navigation, has been producing benefits for several decades now. Money can be invested in more immediate needs or in projects that have a higher media impact, but the solutions brought by such initiatives seldom last so many years.
Exploration opens the mind and awakens imagination: Many of the scientists and astronauts of today grew by looking with their parents how astronauts reached the Moon on TV. Just if one in a million kid souls is touched by the magic of inspiration to produce a new Nobel laureate one day.
Exploring and settling in other planets will help our species propagate, avoiding overpopulation in our home planet: There is a natural limit to the number of individuals of any given species within any ecosystem. What will we do when we reach ours? When will that happen? So we should better find solutions more humane than abortion or preventive sterilisation by then.
Colonisation of other celestial bodies will provide us with vast quantities of natural resources: That would help us to reduce depletion here on Earth and possibly improving the balance between our needs and those of each planetary environment.
By exploring and colonising we improve the odds in favour of our survival by offering redundant habitats: Even a planetary scale cataclism would not wipe us out.
By learning about other celestial bodies we learn about our own: In this way we can understand environmental processes better, leading to a more responsible use and care of our home.
Finding life in the universe: In all likelihood and despite the whole UFO debate, scientifically speaking it is possible that there are other plants harbouring life, and even intelligent, sentient species. Making contact would lead to a much higher understanding of what life is, but by developing interstellar travel capabilities we might be better prepared for something that it is likely to exist independently of our technology and ideas. If those - possible - sentient beings prove non-hostile then we could learn a lot from them, and if they prove to be aggressive towards us, then we will be better prepared to handle such situations, even by military means.
Glory: Fame is attained by doing extraordinary things and for those that seek it, becoming space adventurers would be like reaching the same status that the explorers of the New World.
Money: In an infinite universe there are also infinite opportunities to make a fortune.
Turning knowledge in a quest for itself: Refusing to see the universe would be like rejecting knowledge, and we were not born to live the existence of brutes.