Over a period of time, the requirements of a project can change. Some fields might become obsolete and may no longer be used. In the case of Elasticsearch indexes, the fields that are no longer used are not removed automatically; the mapping remains in the index for all the fields that were ever indexed. Each extra field in the Elasticsearch index carries an overhead; this is especially true if you have hundreds or thousands of fields. If, in your use case, you have a very high number of fields that are not used, it can increase the burden on your cluster.