because exploring mars gives us new facts, which helps us develop and so on.
there are better ways to spend money.
need to expand knowledge about the universe somehow.
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because exploration of space may give us an answer to many fundamental questions, and exploration of mars in particular may give us answers about the origin of mars, whether there was water, and what happened to it if it were there. these and many other answers will satisfy our need for knowledge and maybe even give us answers to other questions which today may look like science fiction.