Purpose and Function of library classification:

We have so far studied the meaning of classification. We also have studied the importance of a call number We would now do well to see what exactly is achieved by classifying documents and arranging them in a systematic way in a library. In the era of the information revolution, the role of libraries in acquiring and organising various types of documents hardly needs any emphasis-Libraries as service institutions acquire documents for use. These acquisitions should systematically be arranged so as to meet the ever growing needs of readers precisely, exhaustively and expeditiously. You have already been told' in sub-sections 2.2.3 and 2.2.5 that if documents are arranged in library on the basis of factors other than subject matter, the arrangement will not be helpful in meeting the requirements of the majority of readers who usually approach a library for subject material. In other words, documents should be classified and arranged on the basis of their subject content.  

We are witness to the information revolution. Documents are published in various languages in various disciplines in diverse forms. Libraries have always been acquiring books and adding them to their collections. Therefore, the collection of an active library continues to grow year after year. Ranganathan compares active and effective libraries with growing organisms,

In an unclassified library, when the collection grows steadily into thousands and lakhs of volumes,  it would be difficult for the library staff to lay hands on a particular document required by a reader. To meet the subject approach of readers the collection must necessarily be classified by subject. In libraries where the collection is arranged by accession number, or author or title, and not by subject, books on the same subject will be scattered throughout the collection. Even if the books are arranged alphabetically -by subject, the resultant sequence will not be helpful, as unrelated material will come together.

This type of sequence of subjects surely is far less useful and will fail to meet the requirements of readers. Alphabetical sequence leads to alphabetical scattering of logically related subjects; as shown in the above example. It is through systematic arrangement that a filiatory sequence or collection of closely related subjects can be achieved. For this we require a scheme of library classification. Here is an example of arranging documents on the basis of Dewey Decimal Classification which brings documents dealing with different aspects of economics systematically one after another at one place in a collection.