Exploration brings in new discoveries, of which even the smallest of these are great achievements in science, and often lead to bigger discoveries. Space exploration has achieved momentous discoveries and there are surely many more to come. When we know so little about ours and other universes and how things came to be, it is imperative we have the best of the best machinery and people on space exploration to find out. 

Yes, eventually, scientists might find a planet that is suitable to live on.  If we continue space exploration, this might come true as in the future we would have better technology and information.

There have not been several useful applications of the expensive vessel despite initial claims. Most experiments are useful only due to its zero gravity state, which although expensive can be replicated on Earth.

Space exploration is a long term investment, not an instantly gratifying machine 

It will give long term benefits, not instant gratification

The direct benefits of outer orbit exploration have been few. No expediton has directly increased life for people.

When thinking of the human species in terms of millions of years, life on Earth increasingly becomes unlikely and space exploration provides the only viable alternative.

The International Space Station costed more than the entire economy of several countries. Millions more are spent in maintenance. This money could have been very useful elsewhere.

To figth problems that are being faced on Earth such as overpopulation, space settlements are a viable solution.

Space exploration is furthering a path for the future of mankind. All investments made today will be felt a thousand times over in the future.

The argument that billions spent on space exploration could be used elsewhere is flawed as space exploration is an investment, which could help solve several other problems down the line