What is a good use case of the omit_norms option in elasticsearch? I could not find adequate explanation in es website.
Norms are a value stored in the index alongside a field, and used for scoring. With the default scoring algorithm, this combines a lengthNorm (which serves to weigh short fields more heavily than long ones), and any field-level boosts. You can see more on the details of it in the Lucene documentation.
The LengthNorm aspect is mostly helpful on proper full-text fields. On more structured fields in which you don't need a field boost, you can safely omit them.