The term frequency normalisation smoothes the dependence between the WithinDocumentTermFrequency and the DocumentLength. (Singhal et. al., 1996) gives the following two reasons for the need for the term frequency normalisation:
The same term usually occurs repeatedly in long documents.
A long document has usually a large size of vocabulary.
As a consequence, an information retrieval system without term frequency normalisation produces biased results and favours long documents.