Why should books not be banned from libraries?

because otherwise there is no point of having a library! and its help full for h/w instead of buying one

I don't believe it is up to the various libraries to impose censorship upon its borrowers by banning books due to the controversial nature of the books in question or any other reason. Any adult person should be allowed to make their own choices as to reading matter. With children, banning the reading of certain books is a matter for their parents/legal guardians to make. Libraries should be able to make books available for people to read, otherwise what is the point of having a library in the first place?