To enable a person to find a book  (or other material) of which either

the author

the title

the subject

is known.

 

To show what the library has

by a given author

on a given subject

in a given kind of literature

 

To assist in the choice of a book (or other material)

as to the edition (bibliographically)

as to its character (literary or topical)

 

These are sometimes summarized as:

 

1) identifying or finding known items;

2) collocating or bringing together related materials;

3) providing sufficient data so that the user can evaluate and select from among related materials.