What is the purpose of natural and social sciences, respectively?
The purpose of science, in general, is to gain more knowledge about the world. This, in turn, leads to the possibility of improving our lives. The purpose of natural sciences is to find out more, as the name implies, about the natural world, while the purpose of social sciences is to find out more about social phenomena. I think, if you asked a whole bunch of scientists, they will say their primary purpose is to gain knowledge. If it can improve the quality of our lives, great, if it can't, it's still worth finding out.
There is, however, a debate about whether the main goal of science is to improve the quality of our lives or to gain knowledge. In other words, if research on a certain topic will not directly or indirectly lead to an improvement of our lives (medicine, technology,...), should we spend money on it? To give a very concrete example - there are tons of examples; I'm just giving one, take a look at this Numberphile video. Mathematicians found out whether 17 clues is the absolute minimum to solve a sudoku in only one way. This seems fairly pointless -in the sense that it won't improve technology, society,..., and at the end of the video, they immediately try to "give an excuse" by stating that the methods used to solve this problem can be used to solve more important questions. Also, in space missions, they often stress that it can improve our lives in some way if they face criticism regarding the high costs. My opinion on this, and I'm stressing this is just opinion, is that every scientific question is a good one and that even if there is no direct or indirect usage of some new knowledge, it often will someday.
What's the point of science in general and do we need it?
It's hard to say what you mean by "need". If you mean by it "need to survive", then the answer is obviously no; Homo Sapiens has survived for thousands of years without science. The rest is a matter of opinion; some would say it has improved our lives, some would say it hasn't or has even made our lives worse.
I would argue, and again, this is just my opinion - which is all I can give to this kind of question, that science has dramatically improved our lives.
Science (see also this article):
- has helped us get rid of a lot of superstition (witch hunts,...)
- has enabled global, fast and almost free communication (and applications like this forum)
- has increased our lifespan dramatically
- has drastically improved medicine (and also got rid of many non-working and even dangerous 'medical treatments')
- has provided us withfast and safe travel
- has drastically improved agriculture
An example of an author who would disagree with me is Jean-Jaques Rousseau. In his Discourse on Science and the Arts (also called the First Discourse), he argues that the arts and sciences are not beneficial to society, but rather add more dependencies and cover up our enslaved state. He was a critic of luxury (which science and the arts make possible), which lead to more inequality, and even argued that it contributed to the corruption of man and moral deterioration. An example of this moral deterioration is that people, who see a scientist, a philosopher, or any other educated man, will try to be like him. In other words, they'll try to be like someone who they are not. They are being dishonest, Rousseau says. The further we push science and the arts, the further we get from who we naturally would be.