I need to write an essay for The Hitchhiker's Guide to the Galaxy, and the theme is "what is the purpose of science? Do you think it’s actually...

That's a tough essay.  I am a bit confused about why the reference to the book is there though.  Based on the essay's themes, none of them explicitly tell you to apply them to the book.    

Your first step is to decide how you feel about those questions.  You don't need long drawn out thoughts . . . yet.  What is the purpose of science?  I always like to think that science is the search for knowledge.  Scientists seek to better understand how and why something works.  To me, that's science's main purpose.  

The next question listed is "Do you think it’s actually helpful?"  You may feel differently, which is fine.  But for me, yes, I think science is actually helpful.  I don't feel that everything that has ever been done scientifically has been useful, but in general, I think that science studies and produces useful knowledge.  A healthy portion of your essay can be focused on this question, because you can go into a lot of specific science knowledge that you think has been useful.  

The final question is "Does it help humanity become better, or does our foolishness or selfishness get in the way of doing science well?" The other half of your main efforts should be here.  In general, I believe that studying science helps humanity to be better.  But does our selfishness get in the way at times? Absolutely.  Humans are not perfect.  I think you should write about where you see human imperfection within scientific studies.  For me, that topic is focused on scientists asking whether or not something can be done before asking if it should be done.  This is when I think that selfishness arises.  A scientist might be seeking out personal glory and recognition and doesn't consider the consequences of his/her studies.  

I believe that you can use that debate to relate your essay to The Hitchhiker's Guide to the Galaxy.  Deep Thought reveals the answer to life, the universe and everything.  The answer is 42.  Awesome!  The problem though is that we don't know what the question is.  Deep Thought was created to find the answer without ever considering the question.  That's what I sometimes feel about science.  It is finding answers to all kinds of things without considering the ethical and moral questions.